Several data races around handlers of QDesktopServices::openUrl()

There are several data races in the handling of URLs and the URL-handler registry. There's a mutex that some functions use, but some don't, there's an ABA problem with the handler registry, and the mechanism by which deleted handlers get automatically unregistered is also flawed, because it relies on QObject::destroyed(), which comes too late to prevent last-minute calls to the (partially or even fully) destroyed handlers.