LibWeb: Implement HTMLImageElement.complete

This was fairly straightforward, although a small part had to be ad-hoc
since we don't yet load images through Fetch.

With this, we can now see annotation labels on deskto.ps :^)

Userland/Libraries/LibWeb/HTML/HTMLImageElement.h

@@ -40,6 +40,9 @@ public:
     unsigned natural_width() const;
     unsigned natural_height() const;
 
+    // https://html.spec.whatwg.org/multipage/embedded-content.html#dom-img-complete
+    bool complete() const;
+
 private:
     HTMLImageElement(DOM::Document&, DOM::QualifiedName);