Remove direct bitcoin calls from qt/optionsmodel.cpp

Summary:
Partial backport of Core PR 10244
https://github.com/bitcoin/bitcoin/pull/10244/commits/c0f2756be517feddacd7c6b89b9faa888b3fef8e
Progress towards T417
Depends on D2813

Test Plan:

make check
bitcoin-qt # sanity checking

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2814