LibWeb: Schedule HTML::EventLoop processing when there are queued tasks

Since we can't simply give HTML::EventLoop control of the whole program,
we have to integrate with Core::EventLoop.

We do this by having a single-shot 0ms Core::Timer that we start when
a task is added to the queue, and restart after processing the queue and
there are still tasks in the queue.

Userland/Libraries/LibWeb/Forward.h

@@ -65,6 +65,7 @@ class CanvasRenderingContext2D;
 class CloseEvent;
 class DOMParser;
 struct EventHandler;
+class EventLoop;
 class HTMLAnchorElement;
 class HTMLAreaElement;
 class HTMLAudioElement;