Support Bluetooth keyboard

[singalen]

There are at least 3 requests for Bluetooth keyboard support as the time of writing, which makes this one of the most popular requests. SDL doesn't support it out-of-box, though.

It can be done, with quite a bit of work:

• https://stackoverflow.com/questions/19235762/how-can-i-support-the-up-and-down-arrow-keys-with-a-bluetooth-keyboard-under-ios
• https://stackoverflow.com/questions/1267560/receive-iphone-keyboard-events/8862814

Let's plan this.

May borrow some code/experience here: https://github.com/terrychou/iVim/blob/master/iVim/VimViewController%2BExternalKeyboard.swift