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