Fix disabled slider highlighting

(cherry picked from commit 4394936392802fb48fd6ccddc385bedd67aa9b31)

scene/gui/slider.cpp

@@ -192,7 +192,7 @@ void Slider::_notification(int p_what) {
 			Ref<StyleBox> style = theme_cache.slider_style;
 			Ref<Texture2D> tick = theme_cache.tick_icon;
 
-			bool highlighted = mouse_inside || has_focus();
+			bool highlighted = editable && (mouse_inside || has_focus());
 			Ref<Texture2D> grabber;
 			if (editable) {
 				if (highlighted) {