Tags should generalize desktops.
To accomplish this, the main node attributes: (type, ratio) would have
to become a dictionary: ((tf1, (type1, ratio1)), (tf2, (type2, ratio2),
...). (`tf<n>` being a tag field.).
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.
