[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:

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