Make information about commands available via the API.

Users can supply a file path and a tag, and
they will get the list of commands that are run
to create an output file with the given tag from
the given input file.
This information is needed by Qt Creator.

Change-Id: I36f92c200e78831733fccc4f7b5f54b8218d6311
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>