Minesweeper: Port to GML compilation

This patch ports minesweeper to GML compilation,
and introduces a few changes made to associated files.

Userland/Games/Minesweeper/CustomGameWidget.h

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2023, the SerenityOS developers
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGUI/Widget.h>
+
+namespace Minesweeper {
+
+class CustomGameWidget : public GUI::Widget {
+    C_OBJECT_ABSTRACT(CustomGameWidget)
+public:
+    static ErrorOr<NonnullRefPtr<CustomGameWidget>> try_create();
+    virtual ~CustomGameWidget() override = default;
+
+private:
+    CustomGameWidget() = default;
+};
+
+}