Update NSIS installation directory, regkey, icons and imagery from Bitcoin Core to Bitcoin ABC

Summary: Updates NSIS installation directory and regkey. Renames image files to indicate bitcoin-abc.

Test Plan:
Build installer
Check Images
Attach Images

Reviewers: #bitcoin_abc, deadalnix, O1 Bitcoin ABC, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, O1 Bitcoin ABC, jasonbcox

Subscribers: jasonbcox, teamcity, deadalnix, schancel

Differential Revision: https://reviews.bitcoinabc.org/D1370