Fix bug in handling the end-of-movie in MoviePlayer.PlayAnimation

In particular, it wasn't recognizing when the movie had been stepped through all the way to the end, was allowing the movie to loop around and play its first frame before noticing it was ready to be removed.
Fix a bug in flump Movie where when a new symbol appeared on a keyframe, it didn't reset it.

Specifically, in flash, removing a symbol, then adding it again later, will always reset that symbol to the start of its own timeline, so flump should do this too. Making the symbol invisible but leaving it on the timeline, however, will NOT reset it.
Add a comment about a limitation of the frame stepping in Movie.
Merge branch 'upstream/master'