This should help with clients requesting window manager actions like
maximizing or minimizing. This is a first version as it only handles
messages of type WM_STATE_CHANGE and _NET_WM_STATE. But ICCCM and EWMH
know some more.

The other direction, setting of properties by the WM, is already
implemented in Rootless.c.

Fixes ArcticaProject/nx-libs#1015