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.

BUG: 425997
1 job for master in 6 minutes and 52 seconds (queued for 16 minutes and 34 seconds)
Status Job ID Name Coverage
  Build
passed linux kf5-qt5 qt5.15 #81450

00:06:52

52.0%