Add state traits via example Checkbox component

[iamnbutler]

WIP

[iamnbutler]

@maxdeviant Any thoughts on the callbacks in the state traits?

I think including them there means they will be required – I don't know if there is a pattern we can use that makes it so that each individual callback may be implemented, but doesn't have to. Do they need to be separated out into their own traits?

[iamnbutler]

Also any thoughts on the naming pattern?

• {state} - returns the current {state}
• set_{state - sets the {state}
• on_{some_action} - {some_action} callback

Also let me know if I'm using callback incorrectly 🤣