Allow Disable and Enable for Slider widget

[ObaraEmmanuel]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

I often use the slider widget as input and it's not possible to disable and enable it out of the box.

Is it possible to construct a solution with the existing API?

The only work around is creating a custom slider widget and that is a huge inconvenience for such a simple feature. It requires one to extend not only the widget but also the renderer to provide the necessary disabled state styling. Things get messy fast.

Describe the solution you'd like to see.

It would be nice if the Slider widget like other input widget could implement the Disableable interface right out the box. I believe it is not such a huge fix, I could even offer to send in a PR for it.

[dweymouth]

Fixed on develop for 2.5.0