This was causing an assertion violation in Ui::PostponeCall.
- Add a generic Core::QtSignalProducer to convert Qt signals to rpl::producer.
- Track event loop nesting inside QtSignalProducer.
- Use QtSignalProducer for QWindow::activeChanged tracking.
In libdispatch crl::on_main() implementation we bypass Application::notify() frame.
So we handle event loop nesting control manually by wrapping all invokations done
through crl::on_main() with the same methods we do from Application::notify().
John Preston