Develop example of window splitter design pattern

[mcking65]

The window splitter design pattern is at:
http://w3c.github.io/aria-practices/#windowsplitter

Note that this issue depends on issue #129.

[lifeiscontent]

@mcking65 any updates?

[ZoeBijl]

Given the lack of updates and my need for this—I’ve self-assigned this one.

Work on this can be followed over at Codepen.

Todo:

• Keyboard support
• Drag support
• Test test test