Use WalletBalances struct in Qt

Summary:
Suggested by John Newbery <john@johnnewbery.com>
https://github.com/bitcoin/bitcoin/pull/10244#discussion_r177504284

Partial backport of Core PR 10244
https://github.com/bitcoin/bitcoin/pull/10244/commits/9a61eed1fcc16ddcedc315045f470e1958b0760b
Progress towards T417
Depends on D2889

Test Plan:
make check
ninja check
bitcoin-qt # sanity checks

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2899