demuxer stream relate api.
fix crash if seek(0) on an auido only stream
use QThread::idealThreadCount instead of av_cpu_count

av_cpu_count is not available for old FFmpeg
change stream(e.g. audio track) support

BUG:
1. continue to play cause bad delay
2. set the stream while player not playing and not loaded
player: change audio track support
dynamically change stream and continue playing

TODO:
clock.pause()
refine stop code
player: fix audio tracks menu number
do not setup codec context after open

setup must be done before avctx open
TODO:
redesign decoder and demuxer. demuxer should only parse stream and get
info, not setup and open decoder.
set stream before open. use default stream if file changed

TODO:
add a test
fix crash if call AVPlayer.stop/duration before file loaded
misc: translation and fix wrong time format for some Qt
update audio track document
region of interest support. gl and xv not ready

TODO:
aspect ratio change if roi changes. setContentRect?
xv region of interest support
gl region of interest support
region of interest example for videogroup
update README: ROI
gl: upload full texture and render part(roi) of it
move some event handling to video renderer widget's filter

bug: videogroup example can not handle key event
new AVDecoder api