move backgroundhints managment in Applet
Summary:
Applet is the more proper way to do it (and will make easier porting
to KF6) this also make it possible for the scripting interface to
access userBackgroundHints
Test Plan: everything behaves the same in plasmashell, the background button in the handle works
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D25742