[PlasmaComponents Menu] Add maximumWidth property

This allows to limit the maximum width of a menu. It can be useful for instance
in task manager where long file names for recent documents will result in gigantic menus.

The property has a RESET method (which is called if you assign "undefined") which will
set it back to QWIDGETSIZE_MAX to remove the size restriction.

Differential Revision: https://phabricator.kde.org/D4257