Fixes #232

Fixes part 1 of #212. There is currently no way to detect that the
symnlink became valid while running, though. The file will be picked up
only on restart

Not if we just happen to have found the g-ir-compiler

Previously that was done by checking for null. There is now a critical in
that case and one should check for the availability (glib/2907)

LHS needs to bee "upnp:class" otherwise this makes no sense

Fixes #104

It seems that despite announcing audio/x-ogg+vorbis support, one of the
two players implemented by a Raumfeld device does not actually play it.

Also unfortunately its user agent is stock GStreamer souphttpsrc. For
now just force everything using souphttpsrc and not overriding its user
agent to receive audio/ogg.

Fixes #231

Some apps seem to need that

Fixes #220

Fixes #228

Fixes #223

Do not do random extension detection on uris that are not files

Fixes #225