Improve the animation track editor drawing

- Draw a background on alternate lines to ease readability of
  animations with many tracks.
- Draw a background on the currently hovered line.
- Use the editor focus stylebox instead of a custom rectangle
  for the focused track.

editor/animation_track_editor.h

@@ -171,6 +171,7 @@ class AnimationTrackEdit : public Control {
 
 	PopupMenu *menu;
 
+	bool hovered = false;
 	bool clicking_on_name = false;
 
 	void _zoom_changed();