Remove keyboard navigation functionality

[jasonwebb]

Currently the Left and Right arrow keys can be used to move between slides, but this functionality is (at best) really unintuitive for real keyboard-only users and (at worst) unusable for screen reader users. Very few carousels have this functionality, and no visible instructions are provided to explain it, so most users would have no reason to believe it exists.

Additionally, the Left and Right arrow keys are already used by the most popular screen readers (JAWS and NVDA) for virtual cursor navigation to move character-by-character through content. Just by using their virtual cursor normally, screen reader users can potentially trigger slide changes - obviously that's a problem!

• Remove keyHandler method.
• Remove all references to keyHandler (on prev/next buttons, slide dots, and slides/track).
• Document this change in the main README.