Support named aliases for multisignature pubkeys

[cwgoes]

Ref #2554 (review)

(and possibly other features - what UX do we want? open to ideas)

[alexanderbez]

Might be of interest @alessio

[alessio]

I'm on this already: fe791ab (the commit message cointains a typo, but code should be simple and self explanatory I hope)

[alessio]

Fixed in #3264