Summary:
As documented via bug reports and online support requests, many users have difficulty learning that window decorations are customizable, because they don't notice or understand the little icon-only button in the bottom-left corner of each theme preview.

This patch centers the buttons and adds text including the theme name , making it obvious what the buttons are for.

Also, clicking on one of the buttons now automatically selects its corresponding theme, because configuring an un-selected theme doesn't make a lot of sense and could lead to user confusion once this UI is more obvious and widely-used.

BUG: 390245

Test Plan:
{F5761897}

- Clicked on the configure buttons; each one selects its parent theme and opens its configuration dialog

Reviewers: #kwin, #plasma, #vdg, cfeck, graesslin

Reviewed By: #kwin, #plasma, graesslin

Subscribers: richardbowen, zzag, kigwana, rkflx, matheusm, fabianr, abetts, Fuchs, graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D11201