[kcmkwin/kwindecoration] Refine drag-and-drop button UI

Summary:
- Reduce the fake titlebar's height so it matches the real titlebar a bit better
- Reduce background opacity while dropping button to diminish visual noise
- Increase label size
- Re-word label for greater clarity
- Remove redundant icon (the text is enough)

BUG: 407109
FIXED-IN: 5.16.0

Test Plan: {F6804068, size=full}

Reviewers: #vdg, #kwin, GB_2, davidedmundson

Reviewed By: #vdg, #kwin, GB_2, davidedmundson

Subscribers: GB_2, abetts, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D20968