avformat/RFC/WIP: add basic timeline support

This commit currently includes support in ffplay and ffmpeg (only
transcoding, not remuxing) only when using -ignore_editlist -1 (FIXME)
as input option.

Comments and tests very welcome here. This patchset will be regularly
reworked and rebased @ https://github.com/ubitux/FFmpeg/compare/edts

The concept is simply this: some demuxers export an opaque timeline, and
users can use avformat_frame_honor_timeline() on the decoded frames to
check if they should be displayed or not. The function will return 0 if
the frame should not be part of the presentation. Otherwise, it will
return 1, with adjusted timestamps if necessary. Later on, it could
truncate large audio frames to get accurate cropping.

Since the timeline structures are completely opaque and users only have
one function to interact with, we can completely change its internals
easily. As a result, the current implementation just mostly follows the
MOV/MP4 edit list structure. We will probably need something smarter for
MKV stuff and friends.

TODO: check if behaviour is actually correct...
TODO: add doxy and various docs
TODO: avformat version bump + document api changes
TODO: figure out the less intrusive way possible to avoid clashes with
      current demuxer behaviour
TODO: add stream copy support in ffmpeg?