Fix window activations handling without event loop nesting.

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.

Telegram/SourceFiles/chat_helpers/tabbed_panel.h

@@ -68,7 +68,6 @@ private:
 		return !_selector;
 	}
 	void showFromSelector();
-	void windowActiveChanged();
 
 	style::margins innerPadding() const;