Added new option to load loopback module

This merge request is from a feature I developed years ago (when you could only submit merge requests via email... urg).

It adds a new feature to load the loopback module. Its quite simple, straight forward but it requires also a change in pulseaudio. This is what I was told in the mailinglist, as the code does not work without a change in the pulseaudio repository. And that is outside my knowledge.

@tanuk said this in one of the mailing list replies:

You need to provide a patch also for org.freedesktop.pulseaudio.gschema.xml (which is in pulseaudio, not paprefs).