lavc: deprecate the use of AVCodecContext.time_base for decoding

When decoding, this field holds the inverse of the framerate that can be
written in the headers for some codecs. Using a field called 'time_base'
for this is very misleading, as there are no timestamps associated with
it. Furthermore, this field is used for a very different purpose during
encoding.

Add a new field, called 'framerate', to replace the use of time_base for
decoding.
lavf: switch to AVCodecContext.framerate for demuxing
avconv: replace AVCodecContext.time_base with framerate