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/Formatter.cpp

@@ -0,0 +1,101 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022, kleines Filmröllchen <filmroellchen@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "Formatter.h"
+#include "Parser.h"
+#include <AK/JsonObject.h>
+#include <AK/JsonValue.h>
+#include <AK/StringBuilder.h>
+
+namespace GUI::GML {
+
+static String format_gml_object(const JsonObject& node, size_t indentation = 0, bool is_inline = false)
+{
+    StringBuilder builder;
+
+    auto indent = [&builder](size_t indentation) {
+        for (size_t i = 0; i < indentation; ++i)
+            builder.append("    ");
+    };
+
+    struct Property {
+        String key;
+        JsonValue value;
+    };
+    Vector<Property> properties;
+    node.for_each_member([&](auto& key, auto& value) {
+        if (key != "class" && key != "layout" && key != "children")
+            properties.append({ key, value });
+        return IterationDecision::Continue;
+    });
+
+    if (!is_inline)
+        indent(indentation);
+    builder.append('@');
+    builder.append(node.get("class").as_string());
+    builder.append(" {\n");
+
+    for (auto& property : properties) {
+        indent(indentation + 1);
+        builder.append(property.key);
+        builder.append(": ");
+        if (property.value.is_array()) {
+            // custom array serialization as AK's doesn't pretty-print
+            // objects and arrays (we only care about arrays (for now))
+            builder.append("[");
+            auto first = true;
+            property.value.as_array().for_each([&](auto& value) {
+                if (!first)
+                    builder.append(", ");
+                first = false;
+                value.serialize(builder);
+            });
+            builder.append("]");
+        } else {
+            property.value.serialize(builder);
+        }
+        builder.append("\n");
+    }
+
+    if (node.has("layout")) {
+        auto layout = node.get("layout").as_object();
+        if (!properties.is_empty())
+            builder.append("\n");
+        indent(indentation + 1);
+        builder.append("layout: ");
+        builder.append(format_gml_object(move(layout), indentation + 1, true));
+    }
+
+    if (node.has("children")) {
+        auto children = node.get("children").as_array();
+        auto first = properties.is_empty() && !node.has("layout");
+        children.for_each([&](auto& value) {
+            if (!first)
+                builder.append("\n");
+            first = false;
+            builder.append(format_gml_object(value.as_object(), indentation + 1));
+        });
+    }
+
+    indent(indentation);
+    builder.append("}\n");
+
+    return builder.to_string();
+}
+
+String format_gml(StringView string)
+{
+    // FIXME: Preserve comments somehow, they're not contained
+    // in the JSON object returned by parse_gml()
+    auto ast = parse_gml(string);
+    if (ast.is_null())
+        return {};
+    VERIFY(ast.is_object());
+    return format_gml_object(ast.as_object());
+}
+
+}