More implicit acceptance of autocompletions

This PR does two things:

1. We implicitly accept the selected completion if it's an identical match to the current token, and there are no other prefix matches.

2. We do a bit more work to wrangle hiding / showing of the help popup properly.

3. We dismiss the popup on `<modifier> + ENTER`, just in case the popup showed in between the time the user has typed some characters, and press 'CMD + ENTER', for example.