[Mac src] PRHOnOffButton

[Peter Hosey]

This is a sliding-switch on/off button, similar to the one in Time
Machine's preference pane, for Mac OS X.

https://bitbucket.org/boredzo/prhonoffbutton/

Features:

- Fully vector-based drawing, so it works at any size and is
resolution-indepedent
- Both a control and a cell, so it works in a table view
(theoretically) or a matrix (tested) as well as stand-alone
- Supports the mixed state
- Accessible: Looks like a checkbox to Accessibility clients
- Supports arrow-key operation (PRHOnOffButton control only)
- BSD-licensed