Meta+Userland: Allow generating C++ initializer code from GML

This does the exact same thing as the runtime initializer,
except it is faster and can catch some errors much earlier.

The code generator includes these important features:
- Automatic include generation where necessary
- Special-casing for TabWidget and ScrollableContainerWidget
- No use of DeprecatedString where possible

Userland/Libraries/LibGUI/GML/AST.h

@@ -188,6 +188,19 @@ public:
         }
     }
 
+    template<FallibleFunction<StringView, NonnullRefPtr<JsonValueNode>> Callback>
+    ErrorOr<void> try_for_each_property(Callback callback) const
+    {
+        for (auto const& child : m_properties) {
+            if (is<KeyValuePair>(child)) {
+                auto const& property = static_cast<KeyValuePair const&>(*child);
+                if (property.key() != "layout" && is<JsonValueNode>(property.value().ptr()))
+                    TRY(callback(property.key(), static_ptr_cast<JsonValueNode>(property.value())));
+            }
+        }
+        return {};
+    }
+
     template<typename Callback>
     void for_each_child_object(Callback callback) const
     {