Add rounded edges to icon so it looks nicer for macOS pre-tahoe

macOS Tahoe with it's liquid glass design now automatically rounds off
rectangular icons. Prior to which, it would just show the full square.
The current logo is square which looks a bit off on older macOS. This
change just rounds the logo off, and makes it a little smaller to match
the correct sizing.

Fixes bug #134

Author: moogman