feat: Image field interop

[hirasso]

This PR makes it easier to switch between the field types "Image" and "FocusPoint":

• if the field is a FocusPoint field and previously was an image field (the value is an ID), construct the default value based on the id:

function load_value( $value, $post_id, $field ) {
	if (is_numeric($value)) {
		return array(
			'id' => $value,
			'top' => 50,
			'left' => 50
		);
	}
	return $value;
}

• if the field is an image field and looks like it was previously a focuspoint field, use the id:

public function load_value_image($value) {
	if (
		$this->has_exact_keys($value, ['id', 'left', 'top'])
		&& is_numeric($value['id'])
		&& wp_attachment_is_image($value['id'])
		) {
		return intval($value['id']);
	}
	return $value;
}