avidemux, qtdemux, dvdec: signal DVCPRO50 codec properly

Submitted by j^

Link to original bug (#526481)

Description

add fourcc for DVCPRO50 to qtdemux
dv5n is DVCPRO50 NTSC and
dv5c is DVCPRO50 PAL

right now only ffdec_dvvideo is able to decode those files properly
but totem chooses dvdec, not sure how to fix that.
playing them via gst-launch works:
gst-launch filesrc location=test.mov ! qtdemux ! ffdec_dvvideo ! ffmpegcolorspace ! xvimagesink