Deposits to different accounts/mixdepths

[AdamISZ]

While I can for sure agree with the UI decision to just serve an address, rather than let the user choose, for deposits, it's too difficult to use Joinmarket as a user, I think, without being able to choose addresses in different mixdepths. The main reason is that in continuous usage you sometimes want to receive into accounts other than account 0 (say you're getting money from someone and you don't want them to send it to a cluster which is associated with some other activity; just an example).