Misc. fixes for pick() and "when this sprite clicked"

[towerofnix]

Fixes #164 and the "when this sprite clicked" generally not functioning at all.

This is basically a hand-picked cherry basket from #146, where the same fixes are made across several commits (of debugging). Thanks to @adroitwhiz for figuring out most of this!

This is a quick patch PR so we can release this fix sooner.

[towerofnix]

@adroitwhiz I'm not sure whether you prefer to merge or rebase, but since all these fixes are already implemented in #164, you should be able to just keep all of your changes and none of these when merging/rebasing and have everything work OK. I hope! My git-fu isn't quite thorough enough :P

I've tested this all works correctly so just getting your code review on principle. Lemme know if there's something obvious I missed 📦

[towerofnix]

Totally reviewed. Yus.