Propagate MovieClip's frame script pending-info upon being added to the stage

Also abstracts the operation of setting a parent and consistently applies `add/removeReference`.

Fixes the situation where a frame script is added to a MovieClip before it is added to the stage, and then the play head doesn't move, so nobody ever knew about the script.
Merge pull request #1481 from tschneidereit/nat

Propagate MovieClip's frame script pending-info upon being added to the stage