fix empty plugin window when switching generic to custom and back.

possibly since 0106047fd2 - show_all() was removed from show_own_window()

gtk2_ardour/processor_box.cc

@@ -3808,7 +3808,7 @@ ProcessorWindowProxy::get (bool create)
 			setup ();
 		}
 	}
-
+	_window->show_all ();
 	return _window;
 }