Fix shift-action modifier in context menu

This is a second attempt to fix 425997 in Dolphin. !79 (merged) was the first attempt and worked as long as the new sub-context menu had the main context menu as its parent. This is not possible for some sub-context menus as described here.

The new attempt is to install an event filter on QApplication as long as the context menu is open. After some testing, it finally works fine.

The same bug still exists on the desktop, so a patch is also needed there.

Commit description:

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