Add borderFocusStyle to box

[icechen128]

The simple addition of a borderFocusStyle addresses the issues mentioned in #692. If there is a better solution or other considerations, please let me know.

[NikSays]

Would also be nice to add BorderFocusColor default value to styles.go, and use it in NewBox definition, so that it can be set for every primitive. if @icechen128 is no longer active, I can do it