Issue #175: Remove unneeded inverting in async serial data decoder.

 - The getDataValue method inverted all signals, but isMark and isSpace
   inverted the value again, meaning a space is always low voltage and a
   mark is always high voltage. The start bit detection also corrected
   this inversion, so a start bit is always a space/low voltage.
 - Then, the data bits themselves are inverted again, which is
   essentially the only inversion necessary.
 - The parity bits used to compare the three-times inverted databits
   with the one-time inverted bit from getDataValue / isExpectedLevel.
   Now that getDataValue no longer inverts, the parity calculation is
   wrong when inverting. This will be fixed later on.
 - Note that the invert setting only effectively inverts the databits.
   This might not be what it intended to do, but that can be corrected
   in a later commit.
Change stop bit handling in async data decoder.

 - Instead of checking each stop bit separately, it now regards the stop
   bits more like a stop period: the value is checked during the first
   bit length and then the length of the stop period is checked by
   finding the next start bit.

   This change prepares for a next change, which has a bit of trouble
   handling the value of half bits properly (which is now no longer
   needed).

 - As an extra bonus, this commit also prevents decoding bytes after
   endOfDecode. The previous code would check if there was still enough
   room for an entire byte after the previous byte, now the check changed
   to see if there is enough room after the start bit, which makes more
   sense.
Introduce DataBitExtractor helper class in the async data decoder.

 - This makes the sampling of bits happen in single place in a
   consistent way, preparing for upcoming commits. No actual behaviour
   should be modified by this commit, effectively only code is moved
   around.
Fix and simplify parity checking in the async data decoder.

 - This was broken a few commits ago by changes in the inverting
   behaviour. Now, we introduce the proper support for inverting bits
   when reading them, instead of as a post-processing step on the
   complete byte, which makes sure the parity bit is also properly
   inverted before checking.
Remove useless check in UARTDataSet.setSampledBitLength().

 - diff is the result of Math.abs, it can never be negative.
Let UARTDataSet calculate its exact baudrate.

 - Previously, this baudrate was set explicitely by UARTAnalyserTask.
   Now it is calculated based on the bit length set by UARTAnalyserTask.
   This also means that the bit length averaging code (e.g., taking the
   average bit length between TX and RX lines) is actually used now,
   which could improve the exact baudrate calculation.
Calculate exact baudrate from the bitlength used by the async data decoder.

 - Previously, the bit length calculated by the BaudRateAnalyzer was
   used for this. However, this bit length was used to calculate the
   baud rate, which was used by the data decoder again to calculate the
   actual bit length to use.
 - This commit should not change any behaviour, since these two bit
   lengths should be the same.
 - This commit prepares for subsequent commits, which allow these values
   to become different and/or allow running without a BaudRateAnalyzer
   at all.
Only use the BaudRateAnalyzer when autodetecting.

 - Before, the analyzer also took care of normalizing the displayed
   baud rate to one of the standard ones and normalizing the baud rate
   passed to the async data decoder to a fraction of the sample rate.
 - Now, when the user enters a non-standard baudrate, it is actually
   displayed as entered (normalization to standard baud rates is still
   used when auto-detecting).
 - Now, when the user enters a non-standard baudrate, it is actually
   passed as entered to the async data decoder (which currently does the
   same fraction of samplerate normalization, but this will change
   next).
Make all bit lengths double in the async data decoder.

 - In addition, the "current time" variable is also made a double.
   Combined, these prevent effectively rounding to the nearest full
   sample time after every bit, which can cause decoding errors when 8
   of those rounding errors are stacked.
 - This commit significantly improves decoding for low samplerates
   (tested with a samplerate of 50Khz and a baudrate of 9600),
   especially when the baudrate is not (near) an integer fraction of the
   samplerate.
Let the async data decoder resync on every edge it sees.

 - This helps to decode signals when there is a small error in the
   baudrate, provided that there are sufficient edges inside the byte.
Let the async data decoder actually measure bit length differences

 - The previous commit made the decoder resync whenever it saw an edge
   in the area where it would expect one. This helps decoding a signal
   where the baudrate / bit length is not entirely correct, but from
   these corrections we can again calculate the real baudrate / bit
   length.

 - This is done by keeping a running total of the number and length of
   bits we find between two confirmed edges, from which we can calculate
   an average at the end, from which the baudrate is calculated again.
Improve the BaudRateAnalyzer autodetection

 - Previously, the most commonly seen length between edges was used as
   the length of one bit. Now, the (weighted) average of all lengths
   near the the most commonly seen length is used, which helps a lot
   when the sampling rate isn't very high (since there is no longer a
   rounding error from rounding to an integer number of samples).
Fix comment on AsyncSerialDataDecoder::findStartBit.
Remove two unused accessors.
Correct comment about SerialConfiguration defaults.
Convert MSB first UART checkbox to a dropdown.
Convert Invert UART checkbox to a dropdown.
Issue #175: Add new "Idle level" option to UART decoder.

 - This option allows changing the idle level from the default HIGH
   level to low, affecting the expected value of the start and stop
   bits. The interpretation of the data and parity bits is unchanged
   (and can be influenced with the existing bit encoding toggle).
Adapt UART analysis tests to previous changes.

 - The inversion-related setting have changed, so use the new setters
   for this.
 - For one test case, the number of detected errors dropped a bit. Note
   that that this isn't just that the new code analyzes the data better,
   since at some points it sees errors where the old code didn't.
   However, the capture contains a lot of noise in some places, which
   means it's not entirely clear what "correct" should be. Manual
   comparison of the testcase shows that the new code doesn't do
   obviously incorrect things, so just updating the test case to the new
   values makes sense.
Merge pull request #176 from matthijskooijman/uart-dev

UART decoder improvements