Remove most direct bitcoin calls from qt/walletmodel.cpp

Summary:
Partial backport of Core PR 10244
https://github.com/bitcoin/bitcoin/pull/10244/commits/a0704a8996bb950ae3c4d5b5a30e9dfe34cde1d3
Progress towards T417
Depends on D2871, D2876

Test Plan:
make check
ninja check
bitcoin-qt # sanity checks; perform transactions and make sure affected dialog boxes behave as expected

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2874