Here you are;
https://github.com/dmotion/kirby-icon
Like I said, it’s really a quick implementation of a very basic idea…
It works for me, so I’m happy with it - but I coded it in less than 10 minutes, so please - don’t expect a full-blown Font Awesome Picker Gallery 
It’s my first custom field (besides the Revolution Slider implementation) - and I created it, copy / pasting an existing field, and modifying it afterwards.