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.