Stacking now involves 3 layers: BELOW, NORMAL and ABOVE.
In each layers, floating windows are stacked above tiled windows.
The *stack* function is now extremely simple: it just inserts an item in
a sorted list, relying on *stack_cmp* to compare clients.
Fullscreen windows are no longer special.
The last desktop and monitor are now deduced from the history.
The stacking order is now independent from the history of the focused
nodes: this prevents hacks on both sides.
All windows are now considered in the stacking algorithm: it prevents
tiled windows from one monitor to appear above the floating windows of
another monitor.
Transfered windows are stacked below the windows of the same kind.
