Add public API to control when context menus shall close

Currently, when a user clicks on a checkbox or radiobutton, QMenuPrivate::activateAction closes the menu, which is private in QMenu.

To control whether the context menu shall be closed or not by a click, it would be convenient if it is configurable or the method for controlling it are accessible from outside of QMenu.

 
Background: Wish for KDE

There are lot's of checkboxes and radiobuttons found in many context menus and submenus or subsubmenus of those menus.
It is very cumbersome to configure many of them as everytime a checkbox or a radiobutton is selected, the whole context menu vanishes.

For the second, third, etc. checkbox/radiobutton the whole process of right clicking, going through the menu/submenu and selecting/deselecting starts over and is not very efficient.

Therefore it would be convenent to keep menu/submenus opened after a click on a radiobutton or checkbox and only close it if the user clicks somewhere outside of the context menu or presses ESC as the user would usually do if no action was triggered.

Please see additionally the discussion at: https://bugs.kde.org/show_bug.cgi?id=413815