Menu: Don't open disabled submenus

Task-number: QTBUG-39384
Change-Id: I5310c6efb0a7045f6f89c6347551a0043d8e0ca6
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Doc: Updated layouts docs with images and snippets

The images and the corresponding snippets
should help the reader understand the Quick Layouts
better.

Task-number: QTBUG-33799
Change-Id: I0a4831f33d99de87fcd7cb68a36844885509bb06
Sanity-Review: Qt Sanity Bot <qt_sanitybot@qt-project.org>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>
Merge remote-tracking branch 'origin/stable' into 5.3

Change-Id: I44e51646f79c71b9ade6cbedd9e22bcae00833c8