Tl multi speed rebased

Dear Martin,

This PR is a rebase of the previous "TL multi speed" PR #230 on top of current master. I think most of the changes are still relevant.

As a reminder, the goal here is to replace occurrences of PhFrame by PhTime in various Joker methods. The most significant changes are related to PhClock, PhVideoEngine and PhSynchronizer. With these changes, a 24 fps video is satisfyingly played together with a 25 fps detX, and hopefully Joker becomes more multi-speed-compatible !

This PR supersedes PRs #214, #216 and #230.

(I believe that a small error was introduced with your recent changes to PhMediaPanel.cpp: the division by 24000 was not done everywhere, and given that a precision of 1 second is likely to be too small for the slider, I removed it.)

Best regards,

Timothée