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/Property.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Function.h>
+#include <AK/JsonValue.h>
+
+namespace GUI {
+
+class Property {
+    AK_MAKE_NONCOPYABLE(Property);
+
+public:
+    Property(DeprecatedString name, Function<JsonValue()> getter, Function<bool(JsonValue const&)> setter = nullptr);
+    ~Property() = default;
+
+    bool set(JsonValue const& value)
+    {
+        if (!m_setter)
+            return false;
+        return m_setter(value);
+    }
+
+    JsonValue get() const
+    {
+        if (!m_getter)
+            return {};
+        return m_getter();
+    }
+
+    DeprecatedString const& name() const { return m_name; }
+    bool is_readonly() const { return !m_setter; }
+
+private:
+    DeprecatedString m_name;
+    Function<JsonValue()> m_getter;
+    Function<bool(JsonValue const&)> m_setter;
+};
+
+}