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/CMakeLists.txt

@@ -47,7 +47,6 @@ set(SOURCES
     GitCommitSyntaxHighlighter.cpp
     GlyphMapWidget.cpp
     GML/AutocompleteProvider.cpp
-    GML/Formatter.cpp
     GML/Lexer.cpp
     GML/Parser.cpp
     GML/SyntaxHighlighter.cpp