Two issues with "toggle script": any hat block should activate its stack when clicked

[tmickel]

• Scripts that start with edge-activated hats can't currently be started by clicking. This is because the hat is evaluated, and the predicate fails. I think we should just skip the hat block when a user explicitly clicks on a stack starting with an edge-activated hat.
• "Untoggle" is based on the top-block of the thread, instead of the "script-block" of the thread. If the thread moves onto a different script, this can lead to weird behavior.