systray: Do not emit notifications when the main window is active

It is natural not to disturb the user in this case, since his attention
is already focused on the application. In addition, it allows to filter
notifications when the number of unread messages decreases.

client/systemtrayicon.cpp

@@ -59,7 +59,7 @@ void SystemTrayIcon::unreadStatsChanged(Quotient::Room* room)
     const auto mode = Settings().get<QString>("UI/notifications", "intrusive");
     if (mode == "none")
         return;
-    if( room->notificationCount() > 0 ) {
+    if (qApp->activeWindow() != nullptr && room->notificationCount() > 0) {
         showMessage(
             //: %1 is the room display name
             tr("Notification in %1").arg(room->displayName()),