New keyboard events not tested under pyglet and VTK

[corranwebster]

I don't have good examples of either backend where we can test keyboard events. As a result the code may not even be syntactically correct, let alone do the right thing.

Ideally, the text field example and some chaco examples with keyboard interactions should be tested under each backend as a minimal smoke-test.

[jwiggins]

Actually, the VTK and pyglet backends are still broken from the kiva refactor. Some additional work would have to be done before they can be tested.

[jwiggins]

#570 removed the Pyglet and VTK backends.