Merge null-safety and add desktop support

[Ernesto Alfonso]

Please, I see it's a pull request that adds null safety... it would be nice if you merge it.

I may also contribute to add Desktop support if you are interested.. this is a great job, don't let it unmantained.

[Ernesto Alfonso]

Please merge the null-safety pull request!!!

[Agustin Villanueva]

You can use the null safety pull request by adding in pubspec.yaml this code

  slider_button:
    git:
      url: https://github.com/julianodportela/Slider-Button

and running flutter pub get in the terminal