Add popover toolbar builder that uses Flutter's new support for an iOS 16+ system toolbar (Resolves #2032)

[matthew-carroll]

Add popover toolbar builder that uses Flutter's new support for an iOS 16+ system toolbar (Resolves #2032)

Also renamed some files for that were still using the unnecessary underscore "_" prefix.

The following code snippet should let you try the new SuperIOSTextField toolbar:

class _iOSSuperTextField extends StatefulWidget {
  const _iOSSuperTextField();

  @override
  State<_iOSSuperTextField> createState() => _iOSSuperTextFieldState();
}

class _iOSSuperTextFieldState extends State<_iOSSuperTextField> {
  final _textFieldKey = GlobalKey();
  final _focusNode = FocusNode();
  final _textController = ImeAttributedTextEditingController(
    controller: AttributedTextEditingController(
      text: AttributedText("This is some text to copy and paste."),
    ),
  );

  @override
  void dispose() {
    _textController.dispose();
    _focusNode.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return TapRegion(
      groupId: "textfields",
      onTapOutside: (_) => _focusNode.unfocus(),
      child: TextFieldBorder(
        focusNode: _focusNode,
        borderBuilder: _borderBuilder,
        child: SuperIOSTextField(
          key: _textFieldKey,
          focusNode: _focusNode,
          tapRegionGroupId: "textfields",
          textController: _textController,
          textStyleBuilder: _textStyleBuilder,
          hintBuilder: _createHintBuilder("Enter single line text..."),
          padding: const EdgeInsets.all(4),
          minLines: 1,
          maxLines: 1,
          caretStyle: CaretStyle().copyWith(color: Colors.red),
          selectionColor: Colors.red,
          handlesColor: Colors.red,
          popoverToolbarBuilder: iOSSystemPopoverToolbarWithBackupFlutterVersion,
        ),
      ),
    );
  }
}

[matthew-carroll]

@BazinC - I added iOS system menu support to SuperIOSTextfield - would you like to try that out on the text field before I add it to SuperEditor?

[angelosilvestre]

LGTM with a single comment.

[angelosilvestre]

Is WithBackupFlutterVersion an appropriate name? I think iOSSystemPopoverToolbarWithFallback would be simpler.