Fix sliders (#116)

[WinXaito]

Fixes #116

Pre-launch Checklist

• I have run dartfmt on all changed files
• I have updated CHANGELOG.md with my changes
• I have run "optimize/organize imports" on all changed files
• I have addressed all analyzer warnings as best I could
• I have added/updated relevant documentation
• I have run flutter pub publish --dry-run and addressed any warnings

As the documentation says:

  /// Whether wrapping the widget in a stack is the approach
  /// that is goind to be used to render the box. If false,
  /// a transparent border is created around the [child] as a
  /// placeholder, and the real border is only displayed when
  /// [focused] is true.
  ///
  /// Using the stack approach is recommended for widgets that
  /// have a defined size (height and width). You should not use
  /// it with widgets that require dragging.
  ///
  /// This property is disabled by default on the following widgets:
  ///   - [Slider]

This property should be true, and, when this is true, the sliders work (and when we focus with keyboard, no layout move are visible)