Fix timestamp errors due to mismatch between doc and video fps

Dear Joker developers,

Timestamp shifts (of 2'30) have been observed when using a detx together with a 24 fps video. These shifts have been tracked down to these places where *frames* are used to set clock positions without specifying the
timecode type. Then a 25 fps clock was assigned a frame from a 24 fps clock, and since frames depend on the fps it produces a wrong time.

Note: maybe a PhFrame should be a specific class with the timecode type as the member, in addition to the frame number ?

Thanks for your attention !