Fix TreeItem button handling

- Fix incorrect tooltip and `get_button_id_at_position()` when column
  title is visible and when RTL layout is used
- Take `button_margin`, `h_separation`, and `item_margin` into account

scene/gui/tree.h

@@ -645,6 +645,8 @@ private:
 
 	TreeItem *_find_item_at_pos(TreeItem *p_item, const Point2 &p_pos, int &r_column, int &h, int &section) const;
 
+	void _find_button_at_pos(const Point2 &p_pos, TreeItem *&r_item, int &r_column, int &r_index) const;
+
 	/*	float drag_speed;
 	float drag_accum;