qtdemux: Improve framerate calculation/guessing

qtdemux: Improve framerate calculation/guessing

Change the way the output framerate is calculated
to ignore the first sample (which is sometimes truncated
in my testing) and use the new gst_video_guess_framerate()
function to recognise common standard framerates better.

Remove the code that was sorting the first 20 sample
durations and then ignoring the result.