lavf: add AV_DISPOSITION_TIMED_THUMBNAILS

doc/APIchanges

doc/ffprobe.xsd

ffprobe.c

libavformat/avformat.h

libavformat/version.h

tests/ref/fate/concat-demuxer-extended-lavf-mxf

tests/ref/fate/concat-demuxer-extended-lavf-mxf_d10

tests/ref/fate/concat-demuxer-simple1-lavf-mxf

tests/ref/fate/concat-demuxer-simple1-lavf-mxf_d10

tests/ref/fate/concat-demuxer-simple2-lavf-ts

tests/ref/fate/ffprobe_compact

tests/ref/fate/ffprobe_csv

tests/ref/fate/ffprobe_default

tests/ref/fate/ffprobe_flat

tests/ref/fate/ffprobe_ini

tests/ref/fate/ffprobe_json

tests/ref/fate/ffprobe_xml