Set accept/escape buttons based on the bound CROSS/CIRCLE buttons.

Please merge https://github.com/hrydgard/native/pull/113 as well if this is accepted.

This pull request removes the hard coding of cancel/confirm buttons(except a select few; we need some as a bare minimum), and allows the UI to be navigated by the user's bound X and O buttons. It also takes into account the user's iButtonPreference, meaning it'll swap O and X accordingly for confirm/cancel, before passing all of these buttons in two vectors, a confirm, and a cancel, into native.