Improve support for AVM1 SWF loading

By several means:
- Implementing support for the AVM1Movie wrapper, which allows loading AVM1 SWFs into AVM2 ones
- Making nested AVM1 SWFs share the same context, as they should
- Untangling some of the event handling code by moving AVM1-specific stuff from Movieclip into AVM1MovieClip
 - Some additional bits and pieces

 Splitting this patch up into multiple smaller ones for the indivual changes is fairly close to impossible, so I won't do it.