KCM look and feel : set proper default values for kwin kdecoration when applying global style

Summary: BUG: 397595

Test Plan:
KCM Look and feel, apply Breeze or Breeze Dark global style, then go to KCM KWin Decoration (Window Decoration), no decoration is highlighted in the grid.
In kwinrc, the group `[org.kde.kdecoration2]` has a `theme=` with no value.

With this patch, applying Breeze or Breeze Dark global theme will write the proper value in kwinrc and the KCM KWin Decoration will show the correct decoration highlighted.

Reviewers: ervin, bport, meven, davidedmundson, #plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27483