Fix bug with client-on-all-desktops moving to a single desktop

When a client was desktop = -1 and moved to desktop = x, it wouldn't be
correctly removed from other desktops, resulting in empty space.
Documentation fixes
Resize clients to null first, on every change

Unfortunately the pixmap issue did _not_ just happen when we violated
min/maxsize.

Among the clients that cause this issue are:

- Firefox

- Steam

vlc only causes problems on minsize.
Release 1.3.2