Ladybird+LibCore: Use QCoreApplication to drive the main Qt event loop

Using QEventLoop works for everything but it breaks *one* little feature
that we care about: automatically quitting the app when all windows have
been closed.

That only works if you drive the outermost main event loop with a
QCoreApplication instead of a QEventLoop. This is unfortunate, as it
complicates our API a little bit, but I'm sure we can think of a way to
make this nicer someday.

In order for QCoreApplication::exec() to process our own
ThreadEventQueue, we now have a zero-timer that we kick whenever new
events are posted to the thread queue.

Userland/Libraries/LibCore/EventLoop.cpp

@@ -170,4 +170,9 @@ bool EventLoop::was_exit_requested() const
     return m_impl->was_exit_requested();
 }
 
+void EventLoop::did_post_event(Badge<Core::ThreadEventQueue>)
+{
+    m_impl->did_post_event();
+}
+
 }