Fix shift-action modifier in context menu

Before this patch, the shift-action modifier in context menus did not
work when a sub-context menu is open, that does not have the main
context menu as its parent.
The new fix installs an event filter on QApplication whenever a new
context menu is opened to make the context menu aware of shift-presses
even when a sub-context menu is in focus.

BUG: 425997
FIXED-IN: 21.04