Replace save|restoreWindowGeometry with Qt functions

Summary:
Backport of Core PR11335
Completes T442 and T318

Co-authored-by: Jason B. Cox <contact@jasonbcox.com>

Test Plan: ninja

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1964