Add a way to launch a browser without using QDesktopService::openUrl()

The QDesktopServices namespace has traditionally been in QtGui, because, well, you need a desktop to use its services. And, on a more pragmatic level, because the platform integration doesn't exist in QtCore, only at the QtGui level.

But protocols like OAuth that require applications to launch the system browser call for the availability of this function also for CLI programs, without the need to link to a 10MiB Gui library just to execute xdg-open <url> for them.

So, try to move the existing classes to QtCore and either leave the platform-helper-backed implementations as just another (QtGui-provided) handler registered with QtCore or move the relevant implementation (as a plug-in?) into QtCore, too.