Add support for _NET_ACTIVE_WINDOW on xcb

Add support for _NET_ACTIVE_WINDOW on xcb

Use a _NET_ACTIVE_WINDOW client message instead of xcb_set_input_focus
for activating toplevel windows on xcb.

According to the Extended Window Manager Hints the right way to activate
a top-level window is using _NET_ACTIVE_WINDOW (when it is supported by
the WM). Other approaches like calling xcb_set_input_focus should be
avoided when possible, since the WM cannot intercept them.

Change-Id: I9be4901f56cbcfb563baf73ccd71ff17a9bdc1d2
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>