Currently, window thumbnails may be placed behind the panel, which
doesn't look good.
With this, the window heap will cover the same area as the maximize
area.
This makes the WindowHeap more robust to layout changes. Currently, the
main issue with it is that thumbnails will be misplaced if the heap's
scene position changes.
Kirigami.ScenePosition automagically keeps track of the item's scene pos.
This effect is meant to be as a replacement for the present windows and
the desktop grid effect. It is written using QML.
So far, this effect implements only the basic features of the present
windows effect. Desktop management features will be added later.
CCBUG: 295775
CCBUG: 303438
