test: Add a dummy unit test to fix 'make test'.

This should be dropped again when the real unit tests
are working again.
connect: Fix filtering of supported devices.

Now that driver options are split from device options, it is not
possible to check the SAMPLERATE option on a driver as it is a device
only option. The SAMPLERATE test still works on some drivers that
didn't have their options split yet, but this won't hold true for long.
The proper way is to check the driver class, which is the only option
you can expect at the driver level for now.
CMakeLists.txt: mingw: Simplify by using Boost_THREADAPI.
Signal: Save and load signal names as UTF-8 strings.

This fixes bug #498.
mingw: Drop libusb0.dll, always use libusb-1.0.
RowItem: Bundled painting parameters into RowItemPaintParams
Trace: Make paint_axis take RowItemPaintParams
RowItemPaintParams: Bundled scale and offset
RowItemParams: Added font and text_height
Renamed pv::data::Analog::get_snapshots and Logic::get_snapshots
DecoderStack: Emancipate from SignalData
SignalData: Added snapshots accessor method
SignalData: Moved start_time into Snapshot
SignalData: Moved samplerate into Snapshot
Snapshot: Renamed to Segment
DecodeTrace: Removed incorrect use of get_text_colour
Trace: Moved get_text_colour into SelectableItem
Cursor: Moved paint_label into TimeMarker
TimeMarker: Added default get_label_rect implementation
TimeMarker: Added get_text