[UX] Pathauto follow-up: implement autocomplete on fields that support tokens (when the user types "[")

This was an idea that @wesruv came up with in https://github.com/backdrop/backdrop-issues/issues/1497#issuecomment-170...

Is it plausible to do auto-complete when the user enters [ in a field that takes tokens? Seems like it'd be a better default way to add tokens and the token browser can act more as a reference.

...very interesting idea, so filing a separate issue for it so not to be forgotten.