From http://standards.freedesktop.org/wm-spec/latest/ar01s05.html:
> _NET_WM_STATE_FULLSCREEN indicates that the window should fill the
> entire screen and have no window decorations. Additionally the Window
> Manager is responsible for restoring the original geometry after a
> switch from fullscreen back to normal window. For example, a
> presentation program would use this hint.
awesome prefers fullscreen internally already. With this patch, the
previous maximized state will be restored after leaving fullscreen mode.
Fixes https://github.com/awesomeWM/awesome/issues/245.
Closes https://github.com/awesomeWM/awesome/pull/418.