LibGUI: Make sure that children are actually widgets

Previously this section of code would blindly add *anything* as a child
as long as it has been registered as an object. Since there is no
guarentee that those objects are, in fact, Widgets, this feels like a
logical fallacy.

For example, up until this change, this is perfectly valid GML:

```
@GUI::Widget {
    layout: @GUI::VerticalBoxLayout {
    }

    @GUI::VerticalBoxLayout {
    }
}
```

What exactly does it do? Who knows! It doesn't seem to *break*, but I
think we can all agree it shouldn't be valid.

Instead, we now actually verify that the registered class inherets from
GUI::Widget before adding it as a child. We also error if it's not,
which should hopefully help new GML writers from forgetting to write
'layout: ' before the layout definition and being confused as to why
it's not working.

Userland/Libraries/LibGUI/Widget.cpp

@@ -1108,6 +1108,7 @@ bool Widget::load_from_json(const JsonObject& json, RefPtr<Core::Object> (*unreg
         });
     }
 
+    auto& widget_class = *Core::ObjectClassRegistration::find("GUI::Widget");
     auto children = json.get("children");
     if (children.is_array()) {
         for (auto& child_json_value : children.as_array().values()) {
@@ -1123,6 +1124,10 @@ bool Widget::load_from_json(const JsonObject& json, RefPtr<Core::Object> (*unreg
             RefPtr<Core::Object> child;
             if (auto* registration = Core::ObjectClassRegistration::find(class_name.as_string())) {
                 child = registration->construct();
+                if (!child || !registration->is_derived_from(widget_class)) {
+                    dbgln("Invalid widget class: '{}'", class_name.to_string());
+                    return false;
+                }
             } else {
                 child = unregistered_child_handler(class_name.as_string());
             }