Integrate the lnf and wallpaper settings with the KCM logic

Summary:
This was currently very much disconnected and broken in quite a few
cases. The main changes here are:

• disabling ConfigPropertyMap autosave behavior
• having the QML integration write directly in the settings objects
• use the settings objects to update the state of the KCM propertly

Further improvement is needed to simplify the handling of the wallpaper
plugin combo, it's still a bit over the place and fragile.

Reviewers: Plasma, mart, davidedmundson, bport, crossi

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25287