Use system tap/click sound for Previous|Next buttons

Hello Simon. I made this minor edit so that the system tap sound is used when the user taps on the Previous | Next field button. I personally don't think the tap sound is necessary for the Done button so I didn't implement that, maybe someone feels differently but that should be a trivial change if desired.

This change honors the user's Keyboard Clicks setting (Settings > Sounds > Keyboard Clicks).

Thank you for your work on this, I've used it both my apps so far (one in app store, one in development).