Typing for GmailBindAction Incorrect if register used

[cancan101]

register allows adding arbitrary DOM listeners which in turn means the strict set of possible actions specified GmailBindAction may not be correct. This means the typing on on_dom, on, etc are overly strict. I don't know a clean way to relax this other than changing the type to string. I suppose as a starting point we could relax the typing on on_dom

[cancan101]

This is how I would fix this: https://github.com/KartikTalwar/gmail.js/pull/700