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.

Ladybird/main.cpp

@@ -56,6 +56,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 
     Core::EventLoop::make_implementation = Ladybird::EventLoopImplementationQt::create;
     Core::EventLoop event_loop;
+    static_cast<Ladybird::EventLoopImplementationQt&>(event_loop.impl()).set_main_loop();
 
     TRY(handle_attached_debugger());