vtl: rework import workflow

vtl: rework import workflow

 - do not allow to simply copy the video file
 - decision to transcode or just reference is made in "transcode dialog"
   not during file-selection.
   Only there the actual codec is known and can be judged to be suitable
   for direct reference
 - simplify "transcode dialog" - no more buttons for specific actions,
   drop-down select actions.
 - increase minimum bitrate for imported file to 500kbit/s