Add "Configure Displays" action to desktop context menu

With a multi-screen setup, it's possible to get into a situation where
all of your means to launch apps have gone missing because a screen
forgot its containmewt or moved the only panel onto a disabled output,
or any number of other issues that unfortunately have not yet been
resolved. In such a situation, it is common for people to try to fix it
by visiting the KScreen KCM to switch around their primary screens,
enable disabled outputs, turn it off and on again, etc.

However if your only visible means to launch apps has gone missing, you
may have a hard time accessing the KScreen KCM. In such a circumstance,
it can be helpful to have the action in the desktop context menu so that
people can right-click on the wallpaper of any screen that *is* working
and make System Settings launch with the KScreen KCM on that screen.

Co-authored-by: Nate Graham <nate@kde.org>

BUG: 355679
FIXED-IN: 5.24