Fix styling of slider and progress bars (alignment with other widgets, baselines)

[SylvainCorlay]

Make the IntSliderand FloatProgress have width @widget-width. Now the widthattribute of Dropdown controls the complete width.

[SylvainCorlay]

@jdfreder A notebook to test alignment: https://gist.github.com/SylvainCorlay/5d4632fe274840222827

[SylvainCorlay]

The next change should be to fix base line alignment between the horizontal [Float/Int]Progress descriptions.

[jdfreder]

I haven't had a chance to try the branch, but the code looks good.

[SylvainCorlay]

This is ready. Fixed the baseline issues in the progress bars - I updated the example notebook gist.

[jdfreder]

Ok, I'll test it - it will be a few minutes - I need to boot another machine.

[SylvainCorlay]

👍 of course one must build the css.

[jdfreder]

Ok, just tested, this looks fantastic! Great work @SylvainCorlay !

[jdfreder]

... I think it would be a good idea to upload the alignment notebook somewhere in the repo - maybe in the examples folder? Or in the tests folder?

[SylvainCorlay]

Yes, that is right. I should probably polish it first, and add some comments...