Clean up kwindecoration KCM qml code

Summary:
Remove all instances of anchors in a layout.
This is undefined behaviour (even if it happens to work) and results in
a warning in 5.11.

Resulted in some shuffling about, but generally cleaner code.

Visually looks the same

Test Plan:
Added some buttons
Dragged and dropped some buttons out of the top header
Still got my "drop here to remove button" hint with the same opacities

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D12284