Tracks elements being edited in its subtree (e.g. a GuardedForm with changes).
May be used to delay a data update while user is editing, so that the input isn't lost when the subtree is recreated.
GuardedForm (and so SubmitFormBuilder) automatically registers to the closest EditingScope when changed.
Nested scopes are supported: an element being edited also marks all ancestor scopes as editing.
Must be placed above the subtree that may be recreated.
Parent scope is resolved once: moving a scope or a GuardedForm to another place in the tree (with a GlobalKey) isn't supported.
Builds builder with value, recreating its subtree (its state is lost) each time value changes,
except while an element is being edited in it (see EditingScope): the displayed value is then kept,
and updated to the latest value when editing ends.
Values are compared with ==: a subtree won't be recreated after a save if the new value equals the previous one.
Inside a FetchBuilder or EventFetchBuilder, keep FetcherConfig.fadeOnDataChange to false (default):
otherwise this widget is recreated on each data and can't keep the displayed value.
Loading and error states also replace this widget (e.g. stream error, refresh with clearDataFirst).
A Form that tracks for changes and may prevent pop.
Current implementation just track if form has been modified once (calling Form.save() doesn't reset status).
Call markAsSaved to reset status (automatically done by SubmitFormBuilder on success).
When changed, registers to the closest EditingScope, until saved or disposed.
Scrolls to the first invalid FormField in the subtree rooted at formContext.
Must be called after Form.validate so that FormFieldState.hasError is up to date.
