Load config specified by the LnF theme

Summary:
This is loaded in a QQuickView like the wallpaper, rest of the code is
mostly based on something quite similar.

Only differences is that wallpapers packages can change during the
config, whereas the base package cannot; which simplifies some code.

BUG: 384264

Test Plan:
Started playing some tunes
Locked screen, still had my controls
Changed setting
Locked screen, didn't have my controls
Changed wallpaper to confirm I hadn't broken anything

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9684