Add a confirmation dialog for display changes

Unfortunately, display configuration can and does fail, due
to unspecified HW constraints, drivers bugs, unsupported exotic
configurations or just bad luck.
So when the user makes a change in the control center, show
a dialog asking him if it looks OK, and revert back after 20 seconds
otherwise.

https://bugzilla.gnome.org/show_bug.cgi?id=706208

src/shell-wm.h

@@ -39,6 +39,9 @@ void     shell_wm_completed_destroy          (ShellWM         *wm,
                                               MetaWindowActor *actor);
 void     shell_wm_completed_switch_workspace (ShellWM         *wm);
 
+void     shell_wm_complete_display_change    (ShellWM         *wm,
+                                              gboolean         ok);
+
 G_END_DECLS
 
 #endif /* __SHELL_WM_H__ */