[electrum] rename icons without whitespaces and capital letters

Summary: We are about to add a linter that checks file names and file permissions, and it does not like capital letters and spaces in filenames.

Test Plan: Run Electrum ABC from sources, check that the CashFusion icons appear as expected in the status bar (green when activated, gray otherwise)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15432