State refactor

[soywod]

The React <StrictMode> revealed inconsistency in the actual implementation. A deep refactor was needed, with the following rules in mind:

• The state should be the source of truth, so it can be easily tracked down. This implies to add information about current values, focused input etc.
• The view (DOM) should reflect the state in a useEffect
• The PIN field should perform the minimum interaction with HTML, DOM and browser.

The actual solution only relies onChange (which should be compatible on most of browsers), on composition events onCompositionStart and onCompositionEnd to prevent composed key to feed the field, and onKeyDown to only detect backspaces. It may fix #80.