media: sort candidates in sipe-media.c

With this change we have only a single place where we do the sorting and
can avoid unnecessary list copies.
media: ignore codecs with duplicate payload type

Buggy(?) codecs may report non-unique id (a.k.a. payload type) this must
not appear in SDP messages we send. Thus, let's ignore any codec having
the same id as one we already have in the converted list.
Revert "media: don't announce Opus codec"

This reverts commit c6b0d23433777337ae81a75af6676a6b7314bde1.

Call now connects even when Opus is offered in SDP. The previous failure
must have had a different cause.