LibGUI: Remove GML prefix in favor of proper namespace

Prefixes are very much a C thing which we don't need in C++. This commit
moves all GML-related classes in LibGUI into the GUI::GML namespace, a
change somewhat overdue.

Userland/Libraries/LibGUI/GML/Parser.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2022, kleines Filmröllchen <filmroellchen@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+
+namespace GUI::GML {
+
+JsonValue parse_gml(StringView);
+
+}