uart: Make data format selection a PD option.

uart: Make data format selection a PD option.

Until now we (ab)used annotation types for outputting the same data
(numbers) in different formats (hex, ascii, binary, and so on).

Turn this into a proper PD option, since annotation types should rather be
used for different _types_ of annotations (e.g. "CRC", "Stop bit",
"Preamble", "Sequence counter", "Warnings", and similar things), not
different _formats_ for the same annotation type.

Old sigrok-cli invocation for hex output:

  sigrok-cli ... -P uart:rx=0:tx=1 -A uart=hex

New:

  sigrok-cli ... -P uart:rx=0:tx=1:format=hex

In GUIs there is now a new "Data format" option where the user can
select the output format for UART data (default is 'ascii').