Do not use macros for accessibility constants

This improves the chance that someone googling for a certain accessibility
attribute or other accessibility constant in order to find inspiration how to
implement it will find our code in OakTextView. It also makes the code more
understandable for reading by someone else, and generally follows the way
others write accessibility code (including Apple in their code samples). It
does make the code more verbose, but I think that does not outweight the above
advantages.

Inspired by: googling and "githubbing" for NSAccessibilityLinkTextAttribute:
http://lists.apple.com/archives/accessibility-dev/2014/Aug/msg00016.html