Optimize PNG images

Summary:
This is the result of the contrib/devtools/optimize-pngs.py script.
This saves the grand total of 14819 bytes.

Test Plan: Check the icons are still good-looking.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3604