decode: add support for Google Trace Event output (JSON)

decode: add support for Google Trace Event output (JSON)

Introduce the --protocol-decoder-jsontrace command line option, which
outputs protocol decoder annotations in the Google Trace Event format.
Resulting files can be viewed by means of the chrome://tracing URL in
the Chrome browser (optionally zipped).

Example use:
  $ sigrok-cli -i $F -P uart -A uart=rx-data --protocol-decoder-jsontrace

The initial implementation was
Submitted-By: Dave Craig <dcraig@brightsign.biz>
This commit introduces generic prepare and close support, should other
output formats need to flush previously accumulated data, or defer the
creation of a header and make sure to add a footer around the sequence
of annotations.