sip: Determine common codecs before answering call

I was running into the same issue as described here and looked into fixing it. Apparently, the preferred codecs were never used while answering incoming calls, but only for outgoing ones. In the outgoing direction, the preferred codecs were respected because of those lines. When answering calls however, self->codecs is used, which wasn't set properly before.

Those changes do allow me to answer incoming calls (with sound), confirmations from others having the same problem would be welcome.

As I'm completely new to the codebase and have never worked on a C GNOME project, I'm completely unsure, whether my changes make sense and are done in the right place. I was especially unsure about when to use g_autoptr and a normal pointer. Clarifications on that welcome!

Fixes (maybe) #413 (closed)