Replace old book icon with new Gehn icon.

Remove unused resources and update the application icon and banner.
Update Gehn logo on loading screen.

We're removing the spinning dots, and have replaced them with
a pulsating opacity on the gradient light-rays.  Also included
is an increase in the number of output frames for the animation.
Merge remote-tracking branch 'deledrius/gehn_resources'

This includes branding updates for Gehn 18 that should
have been a pull request, but aren't because github phailed.