Tl fix video2398fps

Dear Martin,

You will find here a collection of commits to remove more uses of PhFrame. The PhVideoEngine in particular does not use frames anymore, and the conversions in the PhTimeCode class operate preferentially on PhTime too.

I was prompted to do this after observing that videos with exotic framerates (23.976 fps in particular) where showing delays between the video and the strip. I tracked this shifts to the conversions that happen back-and-forth between PhTime and PhFrame.

I hope these commits can make the timing code in Joker more robust !

Thanks again for your work on Joker.

Best regards,

Timothée