LibGUI+LibCore: Move GML property system from LibCore to LibGUI

Since Core::Object properties are really only used by GML now that the
Inspector is long gone, there's no need for these to pollute
Core::Object.

This patch adds a new GUI::Object class to hold properties, and makes
it the new base class of GUI::Window, GUI::Widget and GUI::Layout.
The "instantiate an object by name" mechanism that GML uses is also
hoisted into GUI::Object as well.

Userland/Libraries/LibGUI/Object.cpp

@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2023, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGUI/Object.h>
+
+namespace GUI {
+
+Object::Object(Core::Object* parent)
+    : Core::Object(parent)
+{
+}
+
+Object::~Object() = default;
+
+void Object::register_property(DeprecatedString const& name, Function<JsonValue()> getter, Function<bool(JsonValue const&)> setter)
+{
+    m_properties.set(name, make<Property>(name, move(getter), move(setter)));
+}
+
+JsonValue Object::property(DeprecatedString const& name) const
+{
+    auto it = m_properties.find(name);
+    if (it == m_properties.end())
+        return JsonValue();
+    return it->value->get();
+}
+
+bool Object::set_property(DeprecatedString const& name, JsonValue const& value)
+{
+    auto it = m_properties.find(name);
+    if (it == m_properties.end())
+        return false;
+    return it->value->set(value);
+}
+
+static HashMap<StringView, ObjectClassRegistration*>& object_classes()
+{
+    static HashMap<StringView, ObjectClassRegistration*> s_map;
+    return s_map;
+}
+
+ObjectClassRegistration::ObjectClassRegistration(StringView class_name, Function<ErrorOr<NonnullRefPtr<Object>>()> factory, ObjectClassRegistration* parent_class)
+    : m_class_name(class_name)
+    , m_factory(move(factory))
+    , m_parent_class(parent_class)
+{
+    object_classes().set(class_name, this);
+}
+
+bool ObjectClassRegistration::is_derived_from(ObjectClassRegistration const& base_class) const
+{
+    if (&base_class == this)
+        return true;
+    if (!m_parent_class)
+        return false;
+    return m_parent_class->is_derived_from(base_class);
+}
+
+void ObjectClassRegistration::for_each(Function<void(ObjectClassRegistration const&)> callback)
+{
+    for (auto& it : object_classes()) {
+        callback(*it.value);
+    }
+}
+
+ObjectClassRegistration const* ObjectClassRegistration::find(StringView class_name)
+{
+    return object_classes().get(class_name).value_or(nullptr);
+}
+
+}