Use a common keyboard_layout file and add layouts
This update switches to using a common keyboard_layout.py file, that should be in adafruit_hid (and therefore removed here) at some point in the future. A couple of keyboard layouts have been added from contributions to adafruit_hid, assuming therefore that the license terms allow using it here too. They have not been tested with international keyboards.
US Dvorak from:
adafruit/Adafruit_CircuitPython_HID#60
German win_de_de from:
adafruit/Adafruit_CircuitPython_HID#73