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/ThreadEventQueue.cpp

@@ -62,8 +62,11 @@ ThreadEventQueue::~ThreadEventQueue() = default;
 
 void ThreadEventQueue::post_event(Core::Object& receiver, NonnullOwnPtr<Core::Event> event)
 {
-    Threading::MutexLocker lock(m_private->mutex);
-    m_private->queued_events.empend(receiver, move(event));
+    {
+        Threading::MutexLocker lock(m_private->mutex);
+        m_private->queued_events.empend(receiver, move(event));
+    }
+    Core::EventLoop::current().did_post_event({});
 }
 
 void ThreadEventQueue::add_job(NonnullRefPtr<Promise<NonnullRefPtr<Object>>> promise)