PiGRRL Zero with audio, more switches and a stronger case

This is a bottom case for an Adafruit PiGRRL Zero game console. It is entirely compatible with the default top case and has the following improvement:
- slots for a 28mm diameter speaker (https://www.adafruit.com/product/1890)
- mounting brackets for an audio amplifier (https://www.adafruit.com/product/2130)
- two more holes for on/off switch, labelled for airplane mode and audio on/off
- power icon next to the main switch
- stronger joints for a better connection (from the SNEAKxxATTA