Summary: Bind the keyboard shortcut {key Meta D} to {nav Show Desktop}.

Test Plan:
Apply patch, compile KWin, do `kwin --replace`, reset KWin shortcuts to default values if you've previously changed the Show Desktop's keyboard shortcut

{key Meta D} now shows and hides the desktop.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: filipf, abetts, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D20066