QWindow::alert misbehaving on Cinnamon desktop environment

Calling QWindow::alert on Cinnamon results in the window being raised to front immediately. This is bad, because it means that input can end up in the wrong program if the window is raised while the user is typing.

Cinnamon definitely supports some way of letting entires in the task bar flash, I saw this happen e.g. with Thunderbird and Pidgin. Qt should use that when available.

I have the gtk2 platform theme loaded, so there's no excuse not to integrate with GTK-based DEs.