add seek to ms for demuxer
api change. repeat and position change signal support in AVPlayer etc.

default unit is ms for all AVDemuxer, AVPlayer. previous ones renamed to
xxxUs and xxxF, in unit us and s.
AVPlayer: repeat(get/set/signal), position(get/set/signal), star/stop
position for repeating in a range. seek to position
qml: position and seek api change
examples: remove clockcontrol