GMLCompiler+LibGUI: Add support for object properties and undo hack

Previously the GML compiler did not support object properties such as
`content_widget: @GUI::Widget{}` for GUI::ScrollableContainerWidget;
this commit adds support for such properties by simply calling
`set_<key>(<TProperty>&)` on the object.
This commit also removes the previous hack where
ScrollableContainerWidget was special-cased to have its singular child
used as the content widget; the only GML file using this behaviour was
also changed to be in line with 'proper' GML as handled by the GML
Playground.

Meta/Lagom/Tools/CodeGenerators/GMLCompiler/main.cpp

@@ -313,6 +313,29 @@ static ErrorOr<void> generate_loader_for_object(GUI::GML::Object const& gml_obje
     }));
     generator.appendln("");
 
+    // Object properties
+    size_t current_object_property_index = 0;
+    auto next_object_property_name = [&]() {
+        return String::formatted("{}_property_{}", object_name, current_object_property_index++);
+    };
+    TRY(gml_object.try_for_each_object_property([&](StringView key, NonnullRefPtr<GUI::GML::Object> value) -> ErrorOr<void> {
+        if (key == "layout"sv)
+            return {}; // Layout is handled separately.
+
+        auto property_generator = generator.fork();
+        auto property_variable_name = TRY(next_object_property_name());
+        property_generator.set("property_variable_name", property_variable_name.bytes_as_string_view());
+        property_generator.set("property_class_name", value->name());
+        property_generator.set("key", key);
+        TRY(append(property_generator, "RefPtr<::@property_class_name@> @property_variable_name@;"));
+        TRY(generate_loader_for_object(*value, property_generator.fork(), property_variable_name, indentation + 1, UseObjectConstructor::Yes));
+
+        // Set the property on the object.
+        TRY(append(property_generator, "@object_name@->set_@key@(*@property_variable_name@);"));
+        property_generator.appendln("");
+        return {};
+    }));
+
     // Layout
     if (gml_object.layout_object() != nullptr) {
         TRY(append(generator, "RefPtr<GUI::Layout> layout;"));
@@ -340,10 +363,9 @@ static ErrorOr<void> generate_loader_for_object(GUI::GML::Object const& gml_obje
         TRY(append(child_generator, "RefPtr<::@child_class_name@> @child_variable_name@;"));
         TRY(generate_loader_for_object(child, child_generator.fork(), child_variable_name, indentation + 1, UseObjectConstructor::Yes));
 
-        // Handle the current two special cases of child adding.
-        if (gml_object.name() == "GUI::ScrollableContainerWidget"sv)
-            TRY(append(child_generator, "static_ptr_cast<GUI::ScrollableContainerWidget>(@object_name@)->set_widget(@child_variable_name@);"));
-        else if (gml_object.name() == "GUI::TabWidget"sv)
+        // Handle the current special case of child adding.
+        // FIXME: This should be using the proper API for handling object properties.
+        if (gml_object.name() == "GUI::TabWidget"sv)
             TRY(append(child_generator, "static_ptr_cast<GUI::TabWidget>(@object_name@)->add_widget(*@child_variable_name@);"));
         else
             TRY(append(child_generator, "TRY(@object_name@->try_add_child(*@child_variable_name@));"));

Userland/Libraries/LibGUI/EmojiInputDialog.gml

@@ -24,7 +24,7 @@
     @GUI::ScrollableContainerWidget {
         name: "scrollable_container"
 
-        @GUI::Widget {
+        content_widget: @GUI::Widget {
             name: "emojis"
             layout: @GUI::VerticalBoxLayout {}
         }

Userland/Libraries/LibGUI/GML/AST.h

@@ -188,6 +188,18 @@ public:
         }
     }
 
+    template<typename Callback>
+    void for_each_object_property(Callback callback) const
+    {
+        for (auto const& child : m_properties) {
+            if (is<KeyValuePair>(child)) {
+                auto const& property = static_cast<KeyValuePair const&>(*child);
+                if (is<Object>(property.value().ptr()))
+                    callback(property.key(), static_ptr_cast<Object>(property.value()));
+            }
+        }
+    }
+
     template<FallibleFunction<StringView, NonnullRefPtr<JsonValueNode>> Callback>
     ErrorOr<void> try_for_each_property(Callback callback) const
     {
@@ -201,6 +213,19 @@ public:
         return {};
     }
 
+    template<FallibleFunction<StringView, NonnullRefPtr<Object>> Callback>
+    ErrorOr<void> try_for_each_object_property(Callback callback) const
+    {
+        for (auto const& child : m_properties) {
+            if (is<KeyValuePair>(child)) {
+                auto const& property = static_cast<KeyValuePair const&>(*child);
+                if (is<Object>(property.value().ptr()))
+                    TRY(callback(property.key(), static_ptr_cast<Object>(property.value())));
+            }
+        }
+        return {};
+    }
+
     template<typename Callback>
     void for_each_child_object(Callback callback) const
     {

Userland/Libraries/LibGUI/ScrollableContainerWidget.h

@@ -21,6 +21,9 @@ public:
     GUI::Widget* widget() { return m_widget; }
     GUI::Widget const* widget() const { return m_widget; }
 
+    // GMLCompiler support for the `content_widget` object property.
+    void set_content_widget(GUI::Widget& widget) { set_widget(&widget); }
+
 protected:
     virtual void did_scroll() override;
     virtual void resize_event(GUI::ResizeEvent&) override;