Simple left/right tiling

this is very simplistic and needs more work (see todos in commit messages) but since i'm using it i can also post it here.

There's problems with surfaces using client side decorations with shadows (e.g. gtk) which could be worked around by telling the surface it's maximized https://source.puri.sm/guido.gunther/wlroots/tree/xdg-toplevel-geom but i want to look at the root cause first.