qdoc: Listing group members across modules

qdoc: Listing group members across modules

This is a first attempt at fixing the problem, but
it probably is not the entire solution. The problem
requires adding attributes to the index files and
then reusing them when the index files are read.
The same problem will be affecting the module
lists themselves, but that is not fixed in this
update.

Task-number: QTBUG-28036
Change-Id: I8593d5b9446e51a5204b6c71f8c4f2b63f445972
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>