LibGUI: Move GML parsing and formatting to new AST

This commit introduces a couple of connected changes that are hard to
untangle, unfortunately:
- Parse GML into the AST instead of JSON
- Change the load_from_json API on Widget to load_from_gml_ast
- Remove this same API from Core::Object as it isn't used outside of
  LibGUI and was a workaround for the object registration detection;
  by verifying the objects we're getting and casting we can remove this
  constraint.
- Format GML by calling the formating APIs on the AST itself; remove
  GMLFormatter.cpp as it's not needed anymore.

After this change, GML formatting already respects comments :^)

Userland/Libraries/LibGUI/ScrollableContainerWidget.h

@@ -27,7 +27,7 @@ protected:
 private:
     void update_widget_size();
     void update_widget_position();
-    virtual bool load_from_json(const JsonObject&, RefPtr<Core::Object> (*unregistered_child_handler)(const String&)) override;
+    virtual bool load_from_gml_ast(NonnullRefPtr<GUI::GML::Node> ast, RefPtr<Core::Object> (*unregistered_child_handler)(const String&)) override;
 
     ScrollableContainerWidget();