LibCore: Move event queueing to a per-thread event queue

Instead of juggling events between individual instances of
Core::EventLoop, move queueing and processing to a separate per-thread
queue (ThreadEventQueue).

Userland/Libraries/LibCore/Event.h

@@ -49,6 +49,7 @@ private:
 
 class DeferredInvocationEvent : public Event {
     friend class EventLoop;
+    friend class ThreadEventQueue;
 
 public:
     DeferredInvocationEvent(NonnullRefPtr<DeferredInvocationContext> context, Function<void()> invokee)