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.

Changelog

@@ -2,44 +2,9 @@ Entries are sorted chronologically from oldest to youngest within each release,
 releases are sorted from youngest to oldest.
 
 
-version 0.7:
+version 0.8:
 
 - many many things we forgot because we rather write code than changelogs
-- libmpcodecs video filter support (3 times as many filters than before)
-- mpeg2 aspect ratio dection fixed
-- libxvid aspect pickiness fixed
-- Frame multithreaded decoding
-- E-AC-3 audio encoder
-- ac3enc: add channel coupling support
-- floating-point sample format support to the ac3, eac3, dca, aac, and vorbis decoders.
-- H264/MPEG frame-level multi-threading
-- All av_metadata_* functions renamed to av_dict_* and moved to libavutil
-- 4:4:4 H.264 decoding support
-- 10-bit H.264 optimizations for x86
-- lut, lutrgb, and lutyuv filters added
-- buffersink libavfilter sink added
-- Bump libswscale for recently reported ABI break
-
-
-version 0.7_beta2:
-
-- VP8 frame-multithreading
-- NEON optimizations for VP8
-- Lots of deprecated API cruft removed
-- fft and imdct optimizations for AVX (Sandy Bridge) processors
-- showinfo filter added
-- DPX image encoder
-- SMPTE 302M AES3 audio decoder
-- Apple Core Audio Format muxer
-- 9bit and 10bit per sample support in the H.264 decoder
-- 9bit and 10bit FFV1 encoding / decoding
-- split filter added
-- select filter added
-- sdl output device added
-
-
-version 0.7_beta1:
-
 - WebM support in Matroska de/muxer
 - low overhead Ogg muxing
 - MMS-TCP support
@@ -58,7 +23,7 @@ version 0.7_beta1:
 - RTP depacketization of QDM2
 - ANSI/ASCII art playback system
 - Lego Mindstorms RSO de/muxer
-- libavcore added
+- libavcore added (and subsequently removed)
 - SubRip subtitle file muxer and demuxer
 - Chinese AVS encoding via libxavs
 - ffprobe -show_packets option added
@@ -126,6 +91,37 @@ version 0.7_beta1:
 - AMR-WB encoding via libvo-amrwbenc
 - xWMA demuxer
 - Mobotix MxPEG decoder
+- VP8 frame-multithreading
+- NEON optimizations for VP8
+- Lots of deprecated API cruft removed
+- fft and imdct optimizations for AVX (Sandy Bridge) processors
+- showinfo filter added
+- SMPTE 302M AES3 audio decoder
+- Apple Core Audio Format muxer
+- 9bit and 10bit per sample support in the H.264 decoder
+- 9bit and 10bit FFV1 encoding / decoding
+- split filter added
+- select filter added
+- sdl output device added
+- libmpcodecs video filter support (3 times as many filters than before)
+- mpeg2 aspect ratio dection fixed
+- libxvid aspect pickiness fixed
+- Frame multithreaded decoding
+- E-AC-3 audio encoder
+- ac3enc: add channel coupling support
+- floating-point sample format support to the ac3, eac3, dca, aac, and vorbis decoders.
+- H264/MPEG frame-level multi-threading
+- All av_metadata_* functions renamed to av_dict_* and moved to libavutil
+- 4:4:4 H.264 decoding support
+- 10-bit H.264 optimizations for x86
+- lut, lutrgb, and lutyuv filters added
+- buffersink libavfilter sink added
+- Bump libswscale for recently reported ABI break
+
+
+version 0.7:
+
+- all the changes for 0.8, but keeping API/ABI compatibility with the 0.6 release
 
 
 version 0.6: