Slider Component - feedback configuration

[ciaranlp]

On other question components there is the ability to hide feedback by setting Display Feedback to False this is not the case with slider component.

This should be added to Slider Component.

[moloko]

I think that it just needs adding into properties.schema, example.json and README.md

It's also missing from example.json for all the other question components (MCQ, GMCQ & TextInput)

And missing from the README for MCQ and GMCQ

If you do a PR for Slider I'll sort out the others

[moloko]

PR here: adaptlearning/adapt-contrib-slider#124

[moloko]

done in adaptlearning/adapt-contrib-slider@e0650c1