Improve performance and correctness of the event handling code

Reduce the number of calls on each keystroke by avoiding the
-selectorForInstance + instanceResponds + ... calls

Avoid using NSStrings to compute the selector, it does malloc()s, a simple
buffer + sel_getUid() is faster

Use the NS*FunctionKey values from Foundation instead of hardcoded values.
And now my XVim stops assert()ing when I use F13 by mistake.

Avoid using self.{modifier,character} but access the ivars directly, it's
faster.

Use iswprint() instead of isPrintable, it's supposed to do what's right...
Do not swizzle KV Observing and -dealloc on NSTextView

both sound like a very bad idea, move that stuff to the XVimView where it
belongs. The less we swizzle, the better we are.