compositor: Add an API to focus the stage X window

gnome-shell has traditionally just called XSetInputFocus when wanting to
set the input focus to the stage window, but this might cause strange,
hard-to-reproduce bugs because of an interference with mutter's focus
prediction. Add API to allow gnome-shell to focus the stage window that
also updates mutter's internal focus prediction state.

https://bugzilla.gnome.org/show_bug.cgi?id=700735

src/core/display-private.h

@@ -465,4 +465,9 @@ gboolean meta_display_process_barrier_event (MetaDisplay    *display,
                                              XIBarrierEvent *event);
 #endif /* HAVE_XI23 */
 
+void meta_display_set_input_focus_xwindow (MetaDisplay *display,
+                                           MetaScreen  *screen,
+                                           Window       window,
+                                           guint32      timestamp);
+
 #endif

src/core/display.c

@@ -5806,6 +5806,18 @@ meta_display_request_take_focus (MetaDisplay *display,
                                   timestamp);
 }
 
+void
+meta_display_set_input_focus_xwindow (MetaDisplay *display,
+                                      MetaScreen  *screen,
+                                      Window       window,
+                                      guint32      timestamp)
+{
+  request_xserver_input_focus_change (display,
+                                      screen,
+                                      window,
+                                      timestamp);
+}
+
 void
 meta_display_focus_the_no_focus_window (MetaDisplay *display,
                                         MetaScreen  *screen,