Move the documentation for the classes to their modules.

Move the documentation for the classes to their modules.

All documented classes under src/widgets should now be
under the QtWidgets module in the documentation. The
QtPrintSupport classes should now be under the QtPrintSupport
module in the documentation.

Change-Id: I236b15443b8b93a61578a1d10f52b64dba938f29
Reviewed-on: http://codereview.qt-project.org/5520
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>