Feedback on Matrix Keyboard implementation #1179

Thank you so much for developing gpiozero! I am developing some open source software to control Lionel O-gauge trains from a Raspberry Pi (see here, if interested) and found the gpiozero library a great way to interface buttons, rotary encoders, joysticks, LEDs, etc., to the Pi and control trains, track switches, and operating accessories.

I wanted to use a 4x4 matrix keypad to enter engine numbers. I’ve developed a Composite Device to do so, and would really appreciate some feedback on the code and suggestions on improvements. Here are a few questions I have:

- Is it correct for me to use Button objects for my columns, or should I use DigitalInputDevice (and if so, why?)
- Would it be OK for the value property to return the actual key pressed, or should it always be an int (or int tuple)?
- I’m not sure what the handlers are I’ve bound to the columns. What do they do? Do I need them? 
- Have I handled events correctly? I had a little trouble figuring it out from the code on Github.

Thanks!

-- Dave Swindell

keypad.py