Replace application icon
Summary:
This replaces application image in splash screen, about window, tray
icon and window icon.
Thanks to mengerian for adapting the image.
Test Plan: Manually tested on Mac. Needs testing on other desktop environments.
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D323