Merge #11237: qt: Fixing division by zero in time remaining

Summary:
c8d38abd6 Refactor tipUpdate as per style guide (MeshCollider)
3b69a08c5 Fix division by zero in time remaining (MeshCollider)

Pull request description:

Fixes https://github.com/bitcoin/bitcoin/issues/10291, https://github.com/bitcoin/bitcoin/issues/11265

progressDelta may be 0 (or even negative according to 11265), this checks for that and prints unknown if it is, because we cannot calculate an estimate for the time remaining (would be infinite or negative).

Tree-SHA512: bc5708e5ed6e4670d008219558c5fbb25709bd99a32c98ec39bb74f94a0b7fa058f3d03389ccdd39e6723e6b5b48e34b13ceee7c051c2db631e51d8ec3e1d68c

Backport of Core PR11237

https://github.com/bitcoin/bitcoin/pull/11237/files

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3376