Gstreamer 1.0: merge !29 into gstreamer_python_upgrade

Related to #56 (closed)

Note this change is built on !71 (merged). If you'd like to review this in isolation, check the 2 most recent commits.

At this point all postcoh.c changes are entirely manual. Anytime it's touched, due to rearranging with gstreamer upgrade, there's the same 20+ conflicts every time. So I'm accepting the old version, then manually adding any changes on top.

I've verified that it builds, but not run tests for this MR.