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/GML/Parser.h

@@ -8,9 +8,10 @@
 #pragma once
 
 #include <AK/Forward.h>
+#include <LibGUI/GML/AST.h>
 
 namespace GUI::GML {
 
-JsonValue parse_gml(StringView);
+ErrorOr<NonnullRefPtr<GMLFile>> parse_gml(StringView);
 
 }