Bugfix: don't focus unmapped container on manage

Long description:

A window may become unmapped on manage when an assignment command unmaps
the window, such as moving it to the scratchpad or killing it.

fixes #1283

To apply this patch, use:
curl http://cr.i3wm.org/patch/604/raw.patch | git am

@@ -505,7 +505,7 @@ void manage_window(xcb_window_t window, xcb_get_window_attributes_cookie_t cooki

     /* Defer setting focus after the 'new' event has been sent to ensure the

      * proper window event sequence. */

-    if (set_focus && !nc->window->doesnt_accept_focus) {

+    if (set_focus && !nc->window->doesnt_accept_focus && nc->mapped) {

         DLOG("Now setting focus.\n");

         con_focus(nc);

     }