fixed onSubmitted behavior and public State classes

[wszak]

1. Calls onSubmitted with fixed up value in _handleFocusChanged. Otherwise there's no way to know the spinner value when the user switched to another form field after manually editing the value with keyboard.
2. Removed underscores from State classes names to make them public. This way we can use GlobalKey with SpinBox'es and access their state.

[jpnurmi]

LGTM 👍

[zhang-peter]

As I emphasized on issue #80, the onSubmitted function should be called when the Spinbox is enabled. Calling onSubmitted without proper validation can cause serious issues. This is because when the Spinbox transitions from a disabled state to an enabled state, it triggers the _handleFocusChanged function, which in turn triggers onSubmitted. However, the system may not want to accept this value.

[wszak]

But onSubmitted is called after fixupValue, so the value is validated.
Maybe you could write a test or provide a specific example that demonstrates the issues you mentioned?