decode: Optionally print sample numbers with protocol decoder annotations

decode: Optionally print sample numbers with protocol decoder annotations

Introduce the "--protocol-decoder-samplenum" command line option (no
short form available), which emits sample numbers with textual output
from protocol decoder annotations, regardless of a log level value.

This shall increase usability of sigrok-cli output with external
postprocessors, as the option exclusively adds sample numbers to the
output but does not affect the presence or layout of any other line.

This commit prepends "text" and "meta" annotations with sample number
information. It does not change "binary" output.