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.
base::lambda -> Fn (type alias for std::function).
base::lambda_once -> FnMut (type alias for base::unique_function).
base::lambda_guarded -> crl::guard.
base::lambda_call_type_t -> crl::deduced_call_type.
Use Qt::Dialog instead of Qt::Tool which works better with window
activation / deactivation handling.
Stop displaying the panel on all spaces when the call is established.
John Preston