use FXForms method to tweak props w/o subclassing

Allows one to "tweak some properties of the field cells, without
subclassing them" as described at
https://github.com/nicklockwood/FXForms
Verify that string keys are used as FXForm keyPath tweaks
updated tests for new feature
Merge pull request #1 from GantMan/form-tweak

updated tests for new feature
tune-up test
embrace FXForm properties! :star:
Merge pull request #22 from shreeve/master

use FXForms method to tweak props w/o subclassing