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/ui/rp_widget.h

@@ -11,6 +11,7 @@ https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
 #include <rpl/map.h>
 #include <rpl/distinct_until_changed.h>
 #include "base/unique_qptr.h"
+#include "core/qt_signal_producer.h"
 
 namespace Ui {
 namespace details {
@@ -173,6 +174,25 @@ public:
 		visibilityChangedHook(wasVisible, !this->isHidden());
 	}
 
+	auto windowDeactivateEvents() const {
+		Expects(Widget::window()->windowHandle() != nullptr);
+
+		const auto window = Widget::window()->windowHandle();
+		return Core::QtSignalProducer(
+			window,
+			&QWindow::activeChanged
+		) | rpl::filter([=] {
+			return !window->isActive();
+		});
+	}
+	auto macWindowDeactivateEvents() const {
+#ifdef Q_OS_MAC
+		return windowDeactivateEvents();
+#else // Q_OS_MAC
+		return rpl::never<rpl::empty_value>();
+#endif // Q_OS_MAC
+	}
+
 	~RpWidgetWrap() {
 		base::take(_lifetime);
 		base::take(_eventStreams);