Essential support for radio and checkbox

Supports sticky button, indeterminate radio and indeterminate checkbox.
This requires the following icons in your icon theme.
 * checkbox-checked-symbolic.svg
 * checkbox-mixed-symbolic.svg
 * checkbox-symbolic.svg
 * radio-checked-symbolic.svg
 * radio-mixed-symbolic.svg
 * radio-symbolic.svg
Fallback icons are available in the Adwaita icon theme.
Simplistic Makefile to call sass
The mix command doesn't accept percent