LibGUI: Dynamically process layouts from GML files

There are only two layouts at the moment, but that could (and probably
will) change in the future. Why limit ourselves by hardcoding these
checks when we can do it dynamically instead?

Userland/Libraries/LibGUI/Widget.cpp

@@ -1090,10 +1090,14 @@ bool Widget::load_from_json(const JsonObject& json, RefPtr<Core::Object> (*unreg
             return false;
         }
 
-        if (class_name.to_string() == "GUI::VerticalBoxLayout") {
-            set_layout<GUI::VerticalBoxLayout>();
-        } else if (class_name.to_string() == "GUI::HorizontalBoxLayout") {
-            set_layout<GUI::HorizontalBoxLayout>();
+        auto& layout_class = *Core::ObjectClassRegistration::find("GUI::Layout");
+        if (auto* registration = Core::ObjectClassRegistration::find(class_name.as_string())) {
+            auto layout = registration->construct();
+            if (!layout || !registration->is_derived_from(layout_class)) {
+                dbgln("Invalid layout class: '{}'", class_name.to_string());
+                return false;
+            }
+            set_layout(static_ptr_cast<Layout>(layout).release_nonnull());
         } else {
             dbgln("Unknown layout class: '{}'", class_name.to_string());
             return false;