LibCards: Add a CardGame base class

For now, the only feature of this is that it sets the background colour
from the `Games::Cards::BackgroundColor` config value. Later, other
card game configuration and shared behaviour can go here, to save
duplicating it in each game.

Userland/Libraries/LibCards/CardGame.h

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2022, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibConfig/Listener.h>
+#include <LibGUI/Frame.h>
+
+namespace Cards {
+
+class CardGame
+    : public GUI::Frame
+    , public Config::Listener {
+public:
+    virtual ~CardGame() = default;
+
+    Gfx::Color background_color() const;
+    void set_background_color(Gfx::Color const&);
+
+protected:
+    CardGame();
+
+private:
+    virtual void config_string_did_change(String const& domain, String const& group, String const& key, String const& value) override;
+};
+
+}