Add a "kind" param for StyledCheckbox, allowing designers to choose different styles of checkbox as needed. In addition to the preexisting default kind (now called Solid), there's an Outline kind with a green checkmark and a transparent fill. This is used in the device trust view, since the default checkbox style looks too much like the green "verified" shield and it's awkward to have those next to each other.
* Improve visibility of font size chooser
* Move slider dot sizes into variables
* Use a standard font size instead of percentage in Slider
* Use shorthand for padding in FontScalingPanel
* Change Slider text pos to px to be consistent when font changes
* Cleaner dot size for Slider selection
