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/LibCore/Property.cpp

@@ -1,19 +0,0 @@
-/*
- * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
- * Copyright (c) 2022, the SerenityOS developers.
- *
- * SPDX-License-Identifier: BSD-2-Clause
- */
-
-#include <LibCore/Property.h>
-
-namespace Core {
-
-Property::Property(DeprecatedString name, Function<JsonValue()> getter, Function<bool(JsonValue const&)> setter)
-    : m_name(move(name))
-    , m_getter(move(getter))
-    , m_setter(move(setter))
-{
-}
-
-}