Support Scratch blocks: "stop all", "stop other scripts in sprite"

[towerofnix]

Branching off #18; "stop all" depends on #209.

I'm grouping these in the same issue as they have the overlapping behavior of "stop multiple scripts at once".

Both of these will probably involve filtering/discarding lists of active triggers in some way. Remember that "stop all" also deletes existing clones!

[adroitwhiz]

"Stop all" might need to throw an exception, since it needs to stop the current script in the middle of execution no matter how far down in the call stack we are. My rough idea for implementing it is to have the "stop all" function throw an exception that Trigger.step can catch.