Bugfix: client requests to focus switch workspaces

Long description:

Allow client requests of type _NET_ACTIVE_WINDOW to switch workspaces.
This allows task switchers like skippy-xd to work properly.

http://standards.freedesktop.org/wm-spec/latest/ar01s03.html#idm140251368127856

To apply this patch, use:
curl http://cr.i3wm.org/patch/525/raw.patch | git am

@@ -702,8 +702,9 @@ static void handle_client_message(xcb_client_message_event_t *event) {

         }

         Con *ws = con_get_workspace(con);

-        if (!workspace_is_visible(ws)) {

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

+

+        if (ws == NULL) {

+            DLOG("Window is not being managed, ignoring _NET_ACTIVE_WINDOW\n");

             return;

         }