Support multiple candidate selection

[bdarcus]

In helm-bibtex, when presented with a list of candidates, one can do the following:

• hit control-space to mark multiple candidates
• hit tab to access action options, or return to run default

In my view, this is an almost ideal UX for selecting and acting on multiple candidates (though arguably the selected candidates should then be grouped at the top of the list).

That will then pass multiple keys to the relevant functions, and for the insert-at-point commands, those functions take care of the formatting; in some cases they are comma-separated inline, and in others (like inserting BibTeX entries), they are separated by newlines.

So how to do this here?

The obvious answer is to change the read function to use completing-read-multiple.

But the selectrum UI support for multiple selection is not really optimized for examples like this with very long candidates. Same for icomplete-vertical, which uses a similar UI.

I did, however, submit an enhancement request to selectrum to support this case. No code yet, but I'm optimistic we will see it.

So it would be good to resolve this on our end before this happens, so we can test and contribute feedback.

Seems we would also need to get embark to support multiple (see comment).