Fixup: added an informative column (bgo#732339)

This column may be useful so other extensions that use the
MetadataFixer can give more info about the track being fixed
(current solvers of this extension can fix many tracks per
row, so this may not be useful for them, because using just
a cell to show information about more than one track may be
ugly; but the solvers being developed in b-c-e mostly add
one-fix-per-track solvers, therefore the HasTrackDetails
property will be marked as FALSE by default, but virtual,
so derived classes can override it).

Signed-off-by: Andrés G. Aragoneses <knocte@gmail.com>