uart: allow arbitrary sample positions for UART bit values (1-99%)

uart: allow arbitrary sample positions for UART bit values (1-99%)

The previous implementation of the UART decoder used to sample bit values
strictly at the center position within a bit time. This commit introduces
support to sample bit values at arbitrary positions in the range of 1-99%
of the bit time. This allows to work around glitches in existing captures
as well as using the decoder for UART like protocols which don't sample
bit values at the center position (like EIB aka KNX).

This implementation is incomplete (on purpose). Although this version
improves the ability to extract data from captures, it also introduces
inaccuracies in the annotation positions for non-default values of the
sample point position. Addressing this issue is left for later, assuming
that it'll be a byproduct of another commit series that is being worked
on (general annotation position adjustment and stop bits support).