Various nits in walletview.{h|cpp}

Summary: As per title. Braces, Amount construction, etc...

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1717