Tl multi speed

Dear Martin,

This PR tries to replace uses of PhFrame by PhTime in the various Joker methods. The most significant changes are related to PhClock, PhVideoEngine and Synchronizer. With these changes, a 24 fps video is satisfyingly played together with a 25 fps detX, and hopefully Joker becomes really multi-speed-compatible !

This PR supersedes PRs #214 and #216.

I am wondering whether it is desirable to go one step further and make PhClock::setFrame (in particular) private. What's your opinion ? It is not used in Joker, but in the tests.

Best regards