Recheck fullscreen status on ConfigureNotify

IceWM does not send PropertyNotify when exiting fullscreen. As a
band-aid solution also re-check the fullscreen status on ConfigureNotify
events. We're receiving them already so a few extra checks should not be
noticeable in any case.

Fixes #613

src/x11/x.c

@@ -349,6 +349,7 @@ gboolean x_mainloop_fd_dispatch(GSource *source, GSourceFunc callback, gpointer
                         }
                         /* Explicitly fallthrough. Other PropertyNotify events, e.g. catching
                          * _NET_WM get handled in the Focus(In|Out) section */
+                case ConfigureNotify:
                 case FocusIn:
                 case FocusOut:
                         LOG_D("XEvent: Checking for active screen changes");