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/EventLoopImplementationUnix.h

@@ -31,6 +31,8 @@ public:
     virtual void register_notifier(Notifier&) override;
     virtual void unregister_notifier(Notifier&) override;
 
+    virtual void did_post_event() override;
+
     virtual void unquit() override;
     virtual bool was_exit_requested() const override;
     virtual void notify_forked_and_in_child() override;