keyboard navigation should work when rows are hidden/shown

[Becky Gibson]

the test_expand.html example is not accessible as it uses images with an onclick handler to hide and show additional grid rows. However, when a new row is shown, keyboard navigation breaks - the user can no longer down arrow through the data cells. Keyboard navigation should work with hidden and show rows. In addition the example should be implemented in an accessible manner.

[Nathan Toone]

wontfix - the test_expand.html example is not a good example. It's a total hack - and is intended (mostly) to be replaced by treegrid (which we *want* to fix accessibility...it isn't accessible yet though).