bugfix: don't set input focus if not accepted

http://tronche.com/gui/x/icccm/sec-4.html#s-4.1.7

> Clients using the Globally Active model can only use a SetInputFocus request
> to acquire the input focus when they do not already have it on receipt of one
> of the following events:
> * ButtonPress
> * ButtonRelease
> * Passive-grabbed KeyPress
> * Passive-grabbed KeyRelease

Since managing a window happens on a MapNotify (which is absent from this
list), the window cannot accept input focus, so we should not try to focus
the window at all.

Fixes an issue with xfce4-notifyd which (correctly) declines focus when
we send WM_TAKE_FOCUS, which puts i3 in a state where i3 focus and X
focus are different when a notification appears.

src/manage.c

@@ -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) {
+    if (set_focus && !nc->window->doesnt_accept_focus) {
         DLOG("Now setting focus.\n");
         con_focus(nc);
     }