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/Lexer.h

@@ -0,0 +1,69 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/StringView.h>
+
+namespace GUI::GML {
+
+#define FOR_EACH_TOKEN_TYPE \
+    __TOKEN(Unknown)        \
+    __TOKEN(Comment)        \
+    __TOKEN(ClassMarker)    \
+    __TOKEN(ClassName)      \
+    __TOKEN(LeftCurly)      \
+    __TOKEN(RightCurly)     \
+    __TOKEN(Identifier)     \
+    __TOKEN(Colon)          \
+    __TOKEN(JsonValue)
+
+struct Position {
+    size_t line;
+    size_t column;
+};
+
+struct Token {
+    enum class Type {
+#define __TOKEN(x) x,
+        FOR_EACH_TOKEN_TYPE
+#undef __TOKEN
+    };
+
+    char const* to_string() const
+    {
+        switch (m_type) {
+#define __TOKEN(x) \
+    case Type::x:  \
+        return #x;
+            FOR_EACH_TOKEN_TYPE
+#undef __TOKEN
+        }
+        VERIFY_NOT_REACHED();
+    }
+
+    Type m_type { Type::Unknown };
+    StringView m_view;
+    Position m_start;
+    Position m_end;
+};
+
+class Lexer {
+public:
+    Lexer(StringView);
+
+    Vector<Token> lex();
+
+private:
+    char peek(size_t offset = 0) const;
+    char consume();
+
+    StringView m_input;
+    size_t m_index { 0 };
+    Position m_position { 0, 0 };
+};
+
+}