Fix focus issue when client requests focus in fs mode

Long description:

When in fullscreen mode and another window sends a client request
'_NET_ACTIVE_WINDOW', i3 would stay in fullscreen mode but give focus to
the other window. This patch enforces the fullscreen constraints for
focusing.

To apply this patch, use:
curl http://cr.i3wm.org/patch/278/raw.patch | git am

@@ -676,6 +676,11 @@ static void handle_client_message(xcb_client_message_event_t *event) {

             return;

         }

+        if (!con_fullscreen_permits_focusing(con)) {

+            DLOG("Fullscreen constraints do not permit focusing of the client, ignoring _NET_ACTIVE_WINDOW\n");

+            return;

+        }

+

         Con *ws = con_get_workspace(con);

         if (!workspace_is_visible(ws)) {

             DLOG("Workspace not visible, ignoring _NET_ACTIVE_WINDOW\n");