Pi-Stomp Core: Show processing vs. bypass status on LCD

[rreichenbach]

The original pi-Stomp always had a (white) LED halo on footswitch1 for showing processing vs bypass status, but pi-Stomp Core doesn't pre-suppose any LED's, so we should add an indicator to the LCD.

Meantime workaround: you can play a chord and adjust the volume control. If the volume changes, processing is engaged.

[rreichenbach]

The latest commit includes a "processing" icon on the new LCD "toolbar". It looks like a power indicator and glows green when processing and gray when bypassed. The mode (bypassed or processing) can be toggled by rotating the encoder until the icon is highlighted then clicking. It can also be toggled, as it always has, by long-pressing a footswitch with the "bypass" attribute set in pi-stomp/pistomp/default_config.yml