Move the menubar and the menubar_manager to GimpImageWindow

Also move some of their related code and update other code to
go via gtk_widget_get_toplevel(), but also add some horrid temp
/* FIXME image window */ hacks.

app/display/gimpimagewindow.h

@@ -39,6 +39,7 @@ struct _GimpImageWindow
   GimpUIManager   *menubar_manager;
 
   GtkWidget       *main_vbox;
+  GtkWidget       *menubar;
 
   GdkWindowState   window_state;
 };