Ladybird: Implement a basic Inspector window for the AppKit chrome

This commit includes only fetching the DOM tree from the WebContent
process and displaying it in an NSOutlineView. The displayed tree
includes some basic styling (e.g. colors).

Ladybird/AppKit/UI/LadybirdWebView.mm

@@ -227,6 +227,10 @@ struct HideCursor {
         }
     };
 
+    m_web_view_bridge->on_load_finish = [self](auto const& url) {
+        [self.observer onLoadFinish:url];
+    };
+
     m_web_view_bridge->on_title_change = [self](auto const& title) {
         [self.observer onTitleChange:title];
     };