Support _NET_WM_STATE_FOCUSED

Support _NET_WM_STATE_FOCUSED

_NET_WM_STATE_FOCUSED is set on _NET_WM_STATE to indicate that the
window is focused. It must be set when the window is newly focused and
removed once the window no longer has focus.

> _NET_WM_STATE_FOCUSED indicates whether the window's decorations are
> drawn in an active state. Clients MUST regard it as a read-only hint.
> It cannot be set at map time or changed via a _NET_WM_STATE client
> message.

For example, this is used by GTK applications to show the decoration in
an active or inactive state. This change can be tested by opening a GTK
application (like evince), focusing the window and unfocusing the
window, and observing a change in the window decorations.

Fixes #2273