avformat/dash: move reused API to common file and header file

move from dashenc, move DASHTmplId and dash_fill_tmpl_params to
dash.c, they will be used by dash demuxer and dash muxer.

v2 fixed:
1. rename common file from dashcomm.* to dash.*
Suggested-by: Hendrik Leppkes <h.leppkes@gmail.com>

v3 fixed:
1. rename header file pre defined
2. add ff_ prefix for the internal API
Suggested-by: James Almer <jamrial@gmail.com>

Suggested-by: Timo Rothenpieler <timo@rothenpieler.org>
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Steven Liu <lq@onvideo.cn>