Add missing validation on slider

[MarcSkovMadsen]

Closes #6963

[MarcSkovMadsen]

Complication: Editable Sliders have fixed start and end!

I've added the fix for #6963 including tests. But as you can see this makes a lot of other tests fail.

The reason is that its actually a feature of the editable sliders that you can set a value outside of the start and end!

I would need some guidance on what behavior we would like. I see different options:

• Not wanting to validate values outside bounds.
• Implementing different validations for non-editable and editable sliders
• Changing the api of editable sliders such that values outside start and end are not supported. That would be a breaking change. But as I see the current API is problematic.

Complication: Order of inheritance

You will probably also have to revert the order of inheritance for editable sliders to be able to implement a custom validation in one place for them.