Summary:
Wallpaper plugins were storing their configurations in the same config
group, overwriting each other's values.

Test Plan:
  0. Apply revision D11194 (plasma-desktop).
  1. Right-click the desktop and click Configure Desktop.
  2. Select Wallpaper Type -> Plain Color. Click the chooser button
     and select green. Hit Apply.
  3. Select Wallpaper Type -> Picture of the Day. Click Background
     Color button and select red. Hit Apply.
  4. Select Plain Color type again. The color should still be green.

Reviewers: #plasma, mart, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11195