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/LibCore/Object.h

@@ -181,8 +181,6 @@ public:
     void increment_inspector_count(Badge<InspectorServerConnection>);
     void decrement_inspector_count(Badge<InspectorServerConnection>);
 
-    virtual bool load_from_json(const JsonObject&, RefPtr<Core::Object> (*)(const String&)) { return false; }
-
 protected:
     explicit Object(Object* parent = nullptr);