Fix generic UI sliders w/rangesteps

Leave the user in control while the slider is being dragged.

Previously there was a feedback loop:
User-drags slider -> value changes -> value is rounded
-> slider is updated with rounded value (while the user still drags)

gtk2_ardour/automation_controller.h

@@ -102,6 +102,7 @@ private:
 	sigc::connection                             _screen_update_connection;
 	PBD::ScopedConnectionList                    _changed_connections;
 	bool                                         _ignore_change;
+	bool                                         _grabbed;
 };