Changelog: merge various 0.7* sections into a single 0.8 section

FFmpeg never had distinct alfa/beta 0.7 releases, so having a single
0.8 section with all the changes should be less confusing for the
users, and it is also consistent with what we did with 0.5 and 0.6
releases.

Also add a dedicated 0.7 section, explaining how it differs from 0.8.