Adding drag_value to Slider

[almarklein]

Closes #107

This is an attempt to add a drag_value property to the Slider, such that different callbacks can be applied to the slider being dragged, and the slider being released, as proposed in #107.

Disclaimer: I am not yet familiar with the codebase of this repo, so I just tried to update the new property where the value property was updated.

Todo:

• The value is initially undefined, and I'm not sure how to initialize it to the same value as value.
• Also implement this for RangeSlider.
• update the docstring for updatemode to incorporate the new prop.
• More docs.

[almarklein]

cc @emmanuelle

[emmanuelle]

@alexcjohnson @Marc-Andre-Rivet there are still some todo items in this PR but could you please comment whether the PR is going in the right direction?

[alexcjohnson]

Yes, I think this is reasonable. One possible concern is performance - will there be a problem in very large apps with just having this prop exist even if it isn't used? But I think the hoverData prop in dcc.Graph shows that this is OK.

We should update the docstring for updatemode to incorporate the new prop.

Current changes are only of the src, should this also include changes from after running the built?

No, just src should be changed in this PR. Build artifacts will be updated when we make the next release.

[almarklein]

Oh oh, it looks like I did a git screw-up. I'm closing this in favor of #888. Sorry!