Repair the wallpaper config dialog
Summary:
This is full of mixed bindings/signal handlers for properties and of
broken references in here!
There's likely more to do, but those were the most obvious ones when
working on the screenlocker KCM.
Reviewers: Plasma, mart, davidedmundson, crossi, bport
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D25567