keybindings: Don't unmaximize fullscreen windows

Maximizing fullscreen windows is already not possible but unmaximizing still is. Change it so that a fullscreen window can't be unmaximized, too. Fixes a bug where unmaximizing a fullscreen windows changes the window size but keeps the fullscreen state.

We probably should just make sure that unmaximizing a fullscreen window doesn't end up in a weird state on wayland but I didn't manage to.

Fixes: #2212