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.cpp

@@ -0,0 +1,141 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2022, kleines Filmröllchen <filmroellchen@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "Parser.h"
+#include "Lexer.h"
+#include <AK/GenericLexer.h>
+#include <AK/JsonObject.h>
+#include <AK/JsonValue.h>
+#include <AK/Queue.h>
+
+namespace GUI::GML {
+
+static Optional<JsonValue> parse_core_object(Queue<Token>& tokens)
+{
+    JsonObject object;
+    JsonArray children;
+
+    auto peek = [&] {
+        if (tokens.is_empty())
+            return Token::Type::Unknown;
+        return tokens.head().m_type;
+    };
+
+    while (peek() == Token::Type::Comment)
+        tokens.dequeue();
+
+    if (peek() != Token::Type::ClassMarker) {
+        dbgln("Expected class marker");
+        return {};
+    }
+
+    tokens.dequeue();
+
+    if (peek() != Token::Type::ClassName) {
+        dbgln("Expected class name");
+        return {};
+    }
+
+    auto class_name = tokens.dequeue();
+    object.set("class", JsonValue(class_name.m_view));
+
+    if (peek() != Token::Type::LeftCurly) {
+        // Empty object
+        return object;
+    }
+    tokens.dequeue();
+
+    for (;;) {
+        if (peek() == Token::Type::RightCurly) {
+            // End of object
+            break;
+        }
+
+        if (peek() == Token::Type::ClassMarker) {
+            // It's a child object.
+            auto value = parse_core_object(tokens);
+            if (!value.has_value()) {
+                dbgln("Parsing child object failed");
+                return {};
+            }
+            if (!value.value().is_object()) {
+                dbgln("Expected child to be Core::Object");
+                return {};
+            }
+            children.append(value.release_value());
+        } else if (peek() == Token::Type::Identifier) {
+            // It's a property.
+            auto property_name = tokens.dequeue();
+
+            if (property_name.m_view.is_empty()) {
+                dbgln("Expected non-empty property name");
+                return {};
+            }
+
+            if (peek() != Token::Type::Colon) {
+                dbgln("Expected ':'");
+                return {};
+            }
+            tokens.dequeue();
+
+            JsonValue value;
+            if (peek() == Token::Type::ClassMarker) {
+                auto parsed_value = parse_core_object(tokens);
+                if (!parsed_value.has_value())
+                    return {};
+                if (!parsed_value.value().is_object()) {
+                    dbgln("Expected property to be Core::Object");
+                    return {};
+                }
+                value = parsed_value.release_value();
+            } else if (peek() == Token::Type::JsonValue) {
+                auto value_string = tokens.dequeue();
+                auto parsed_value = JsonValue::from_string(value_string.m_view);
+                if (parsed_value.is_error()) {
+                    dbgln("Expected property to be JSON value");
+                    return {};
+                }
+                value = parsed_value.release_value();
+            }
+            object.set(property_name.m_view, move(value));
+        } else if (peek() == Token::Type::Comment) {
+            tokens.dequeue();
+        } else {
+            dbgln("Expected child, property, comment, or }}");
+            return {};
+        }
+    }
+
+    if (peek() != Token::Type::RightCurly) {
+        dbgln("Expected }}");
+        return {};
+    }
+    tokens.dequeue();
+
+    if (!children.is_empty())
+        object.set("children", move(children));
+
+    return object;
+}
+
+JsonValue parse_gml(StringView string)
+{
+    auto lexer = Lexer(string);
+
+    Queue<Token> tokens;
+    for (auto& token : lexer.lex())
+        tokens.enqueue(token);
+
+    auto root = parse_core_object(tokens);
+
+    if (!root.has_value())
+        return JsonValue();
+
+    return root.release_value();
+}
+
+}