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