Clean up kwindecoration KCM qml code
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
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
Differential Revision: https://phabricator.kde.org/D12284