qdoc: Briefs from other modules now show up

The brief text for a documented thing is now output as an
attribute of that thing in the module's index file, and it
is reconstituted in the thing's tree node, when qdoc reads
the module's index file later. Only the verbatim text of
the brief is saved in the index file, i.e. no links or
other markup.

The effect is that brief texts can be used in other modules.

Task-number: QTBUG-31021
Change-Id: I932a0c85259b6d1901138f0c0959ddb9815b7db5
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>