_NET_ACTIVE_WINDOW:
> The window ID of the currently active window or None if no window has
> the focus.
This fixes a bug that would not update _NET_ACTIVE_WINDOW when focus
changed to an i3 container without a window such as a branch or
workspace content container.
Allow client requests of type _NET_ACTIVE_WINDOW to switch workspaces if
they indicate they are a pager. Otherwise, set the urgency hint on that
con to indicate that something happened.
This allows task switchers like skippy-xd to work properly.
http://standards.freedesktop.org/wm-spec/latest/ar01s03.html#idm140251368127856
