The QML changes make the tab look and feel less awful:
- make the fake window borders stand out less
- add margins to drop area to make it easier to drop buttons on the title bar (fixing 346222)
- change cursor to a non-pointing one 
 Drag&drop: When dropping, we receive the position of the button being dropped,
not of the cursor. The two can be far (when starting drag by moving the cursor fast):
which makes the whole experience very confusing, as the user means to drop at cursor location.