ContextMenu: Allow plugins submenus to react to Shift modifiers
In 9e4642d4 support was added for the NewFileMenu. This adds support for the Plugins submenu provided they relay their parentWidget to their own menus.
ark's KAbstractFileItemActionPlugin works but others need to be fixed to relay their parentWidget to their menu. Fix for activity linking at network/kio-extras!57 (merged)
Full context at !79 (9e4642d4)