Don't show applet handle outside of desktop

Summary:
This makes sure the applet handle won't be displayed outside
of available desktop area (under panels or outside of screen).
Also prevent showing the handle under other applets.

BUG: 353992

Reviewers: Plasma, mart

Reviewed By: mart

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D948