* Make FlAccessibleNode derivable
This allows introducing specialized subclasses that implement such a11y
interfaces as AtkEditableText that should not be implemented for the
generic node.
* FlAccessibleNode: add set value, text selection & perform action
Prepare API required for the upcoming AtkEditableText implementation.
* FlViewAccessible: postpone child node creation
This changes the a11y node creation flow so that (unknown) child nodes
are not pre-created when their parent is updated but "pending" child
node IDs are collected for later. Parent-child relationships are
established at the end of update batches when all nodes have been
created.
* Add FlAccessibleTextField that implements AtkEditableText
Solves: flutter/flutter#103191
433ac149bf