(PhVideoEngine) Switch the video engine to PhTime

This fixes another bunch of strip delays with 23.976 fps videos
due to back-and-forth conversions between frames and times in the
engine.
Fix tests following removal of PhVideoEngine::setFrameIn
Fix VideoTest following removal of PhVideoEngin::frameIn and frameLength
(PhVideoEngine) Handle undefined framerate, default to 25 fps
New spec to test default framerate of the video engine