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.h

@@ -19,6 +19,7 @@
 namespace Core {
 
 class EventLoopImplementation;
+class ThreadEventQueue;
 
 // The event loop enables asynchronous (not parallel or multi-threaded) computing by efficiently handling events from various sources.
 // Event loops are most important for GUI programs, where the various GUI updates and action callbacks run on the EventLoop,
@@ -95,6 +96,9 @@ public:
 
     static Function<NonnullOwnPtr<EventLoopImplementation>()> make_implementation;
 
+    void did_post_event(Badge<ThreadEventQueue>);
+    EventLoopImplementation& impl() { return *m_impl; }
+
 private:
     void wait_for_event(WaitMode);
     Optional<Time> get_next_timer_expiration();