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-private.h

@@ -38,6 +38,8 @@ void _shell_wm_kill_switch_workspace (ShellWM             *wm);
 gboolean _shell_wm_filter_keybinding (ShellWM             *wm,
                                       MetaKeyBinding      *binding);
 
+void _shell_wm_confirm_display_change (ShellWM            *wm);
+
 G_END_DECLS
 
 #endif /* __SHELL_WM_PRIVATE_H__ */