Add form navigation support

[marbetschar]

SplitRow should support the navigation with the keyboard accessory buttons <, > - as well as the Next button.

[marbetschar]

firstResponder support is now added. Eureka does not seem to have a facility which can be extended to allow navigating multiple rows embedded in a parent one.

That said, it is now possible to navigate from top to bottom (direction == .down) to activate the left row and from bottom to top (direction == .up) to activate the right row. Unfortunately the user is catched afterwards.