RadioButton use on_change callback

[Bumber64]

Have RadioButton use a callback when it changes state instead of needing to check manually.

[Bumber64]

Not sure if toggle_state should have a getter function. Maybe there's a reason to check the state outside the callback, but you can just read the value if you want it enough.

[SilasD]

I think this covers all of my quibbles. I'm good with this, asuming it's had a bit of testing.

Edit: in response to the above, I think a getState() to match the setState() would be good -- if I were just trying to use this, I would expect the matching function to exist.