ffmpeg: switch to default versioning-scheme
ffmpeg: order and cleanup versions
ffmpeg: change default 0.10 -> 2.0

This commit, and the "pinnings" that follow should not cause major issues.
I personally checked (build-only) each and every package that depends
on ffmpeg to find the maximum supported version.
I performed this on stdenv-updates+glibc2.18+x-updates but I don't expect
that to matter.
gst-ffmpeg: stick to ffmpeg 0.10
xpra: depend on ffmpeg instead of ffmpeg_1, now default became 1+
acoustid_fingerprinter: pin to ffmpeg_1
cfdg: use default/latest ffmpeg
minidlna: pin to ffmpeg 0.10
openscenegraph: pin to ffmpeg 0.10
xbmc: pin to ffmpeg 1
baresip: pin to ffmpeg 1
xinelib: pin to ffmpeg 1
grass: pin to ffmpeg 0.10
renpy: use default/latest ffmpeg
mediastreamer: use default/latest ffmpeg
mlt: use default/latest ffmpeg
opencv: use default/latest ffmpeg
mediatomb: use default/latest ffmpeg
audacity: pin to ffmpeg 0.10
freerdp: pin to ffmpeg 1