Adds backlight support to Adafruit's Capacitive Touch PiTFT (PID: 1983).

Simple way to add support for backlight control on the capacitive touch version of Adafruit's PiTFT.
I'm not great at python but this has been tested and works on my PiTFT.

Conditions tested:
If my screen is off, when I start OctoPiPanel, it turns it on.
If the timeout is reached, it turns if off.
If I touch the screen, it turns it on.
And if I quit OctoPiPanel while the screen is off, it turns it on before quitting.