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.cpp

@@ -0,0 +1,154 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "Lexer.h"
+#include <AK/CharacterTypes.h>
+#include <AK/Vector.h>
+
+namespace GUI::GML {
+
+Lexer::Lexer(StringView input)
+    : m_input(input)
+{
+}
+
+char Lexer::peek(size_t offset) const
+{
+    if ((m_index + offset) >= m_input.length())
+        return 0;
+    return m_input[m_index + offset];
+}
+
+char Lexer::consume()
+{
+    VERIFY(m_index < m_input.length());
+    char ch = m_input[m_index++];
+    if (ch == '\n') {
+        m_position.line++;
+        m_position.column = 0;
+    } else {
+        m_position.column++;
+    }
+    return ch;
+}
+
+constexpr bool is_valid_identifier_start(char ch)
+{
+    return is_ascii_alpha(ch) || ch == '_';
+}
+
+constexpr bool is_valid_identifier_character(char ch)
+{
+    return is_ascii_alphanumeric(ch) || ch == '_';
+}
+
+constexpr bool is_valid_class_character(char ch)
+{
+    return is_ascii_alphanumeric(ch) || ch == '_' || ch == ':';
+}
+
+Vector<Token> Lexer::lex()
+{
+    Vector<Token> tokens;
+
+    size_t token_start_index = 0;
+    Position token_start_position;
+
+    auto begin_token = [&] {
+        token_start_index = m_index;
+        token_start_position = m_position;
+    };
+
+    auto commit_token = [&](auto type) {
+        Token token;
+        token.m_view = m_input.substring_view(token_start_index, m_index - token_start_index);
+        token.m_type = type;
+        token.m_start = token_start_position;
+        token.m_end = m_position;
+        tokens.append(token);
+    };
+
+    auto consume_class = [&] {
+        begin_token();
+        consume();
+        commit_token(Token::Type::ClassMarker);
+        begin_token();
+        while (is_valid_class_character(peek()))
+            consume();
+        commit_token(Token::Type::ClassName);
+    };
+
+    while (m_index < m_input.length()) {
+        if (is_ascii_space(peek(0))) {
+            begin_token();
+            while (is_ascii_space(peek()))
+                consume();
+            continue;
+        }
+
+        // C++ style comments
+        if (peek(0) && peek(0) == '/' && peek(1) == '/') {
+            begin_token();
+            while (peek() && peek() != '\n')
+                consume();
+            commit_token(Token::Type::Comment);
+            continue;
+        }
+
+        if (peek(0) == '{') {
+            begin_token();
+            consume();
+            commit_token(Token::Type::LeftCurly);
+            continue;
+        }
+
+        if (peek(0) == '}') {
+            begin_token();
+            consume();
+            commit_token(Token::Type::RightCurly);
+            continue;
+        }
+
+        if (peek(0) == '@') {
+            consume_class();
+            continue;
+        }
+
+        if (is_valid_identifier_start(peek(0))) {
+            begin_token();
+            consume();
+            while (is_valid_identifier_character(peek(0)))
+                consume();
+            commit_token(Token::Type::Identifier);
+            continue;
+        }
+
+        if (peek(0) == ':') {
+            begin_token();
+            consume();
+            commit_token(Token::Type::Colon);
+
+            while (is_ascii_space(peek()))
+                consume();
+
+            if (peek(0) == '@') {
+                consume_class();
+            } else {
+                begin_token();
+                while (peek() && peek() != '\n')
+                    consume();
+                commit_token(Token::Type::JsonValue);
+            }
+            continue;
+        }
+
+        consume();
+        commit_token(Token::Type::Unknown);
+    }
+    return tokens;
+}
+
+}