Fix problem with MoveTransition and multiple interactions.

The problem would manifest when we did something like
$ renpy.transition(move), and restarted the interaction. We had been
creating a MultiBox with a layers dict, which would adjust the times
for its children. When shown as part of the second interaction, the
times would be reset to 0, and the move would appear to restart.

By getting rid of the layers dict, we prevent this the adjustment
of times from occuring, and things work as expected.