Make gst::Seqnum, gst::MetaSeqnum similar to Gst::ClockTime and represent invalid values in the types

The following discussion from gst-plugins-rs!90 (merged) should be addressed:

• @slomo started a discussion: (+2 comments)

  I wonder if we should change the type definition of that so that seqnums (and group ids, etc) are never 0 == INVALID, and instead use Options in the code. Unrelated to your changes here of course, but seems worth doing.

  Basically changing gst::Seqnum from u32 to a NonZeroU32, and changing the API that can take invalid seqnum (is there any at all?) to an Option<gst::Seqnum>.