Partial fix #2 for FM-719. This fix ensures that when seeking in and out of the child of a parallel element, that the play state is updated according to the play state of the composition. For example, if there are two parallel children of duration 10 and 20 seconds, and the parallel element is playing, and you seek from 15 to 5, then the shorter child should resume playback. (And similar for when paused.) Bug detected when verifying the (soon to be checked in) fix for FM-719, and seeking in and out of the section of video where the image should be displayed. Added unit tests for these cases as well.
