Form: Move out field handlers into subscription functions

[kettanaito]

Why?

Field handlers that exist as Form methods now do not need to access the form (except of to pass the form reference as the form argument). They can be isolated into own functions.

// Form.jsx
constructor() {
+ fromEvent('fieldChange').subscribe(handleFieldChange(this.eventEmitter, this))
}

- handleFieldChange() {...}
- handleFieldFocus() {...}
- handleFieldBlur() {...}

Subscription example:.

// utils/subscriptions/handleFieldChange.js
const handleFieldChange = (eventEmitter, form) = (eventPayload) => {
  const nextFieldState = foo(...)
  eventEmitter.emit('applyStatePatch', ..., ...)
}