Add proposal_id field to VoteInfo structure

[ueco-jb]

https://github.com/CosmWasm/cw-plus/blob/main/packages/cw3/src/query.rs#L101-L105

Info about vote without proposal's context makes little sense, and adding it is really easy.
Also it will be usefull in implementing list_votes_by_voters query, which lists all votes for given voter: check confio/poe-contracts#78