keyboard accessibility - treegrid example + selector not toggle-able via keyboard

[ktiedt]

In the treegrid test case with the selector in the "Visited" column the checkbox is not accessible via the keyboard at all from what I have been able to tell.

At least in OSX Mavericks + latest Chrome

[kfranqueiro]

Turns out there was a 2-year-old regression which occurred between 0.2 and 0.3. Previously, the Keyboard mixin would focus dgrid/editor-related input fields directly, but a change I made unwittingly had the side effect of rendering that code unreachable.

I've restored that functionality, but it apparently never worked in IE8, so I'm inclined to leave this issue open until we can try to figure out why that is (though we probably won't get around to that immediately).