[Accessibility] Process non-focusable windows (popups, menus) as part of the parent window tree.

scene/main/window.h

@@ -332,6 +332,8 @@ public:
 	void set_flag(Flags p_flag, bool p_enabled);
 	bool get_flag(Flags p_flag) const;
 
+	bool is_popup() const;
+
 	bool is_maximize_allowed() const;
 
 	void request_attention();
@@ -399,6 +401,7 @@ public:
 	Window *get_exclusive_child() const { return exclusive_child; }
 	HashSet<Window *> get_transient_children() const { return transient_children; }
 	Window *get_parent_visible_window() const;
+	Window *get_non_popup_window() const;
 	Viewport *get_parent_viewport() const;
 
 	virtual void popup(const Rect2i &p_screen_rect = Rect2i());