Ipywidgets: Allow controlling readout width in Valid widget

[pbugnion]

The Valid widget does not provide a way to control the readout style, as far as I can tell. This was originally raised in issue #1937 .

One way to do this would be to create a new ValidStyle object for the Valid widget that contains both the description_width and the readout_width fields. The ValidStyle object should probably inherit from DescriptionStyle.

Alternatively, we could make a new ReadoutStyle widget, with a corresponding readout_style traitlet on the Valid widget. This seems a bit more extensible in the future.

For a minimal example of the current (broken) behaviour:

import ipywidgets as widgets

widget = widgets.Valid(
    readout='some very very long sentence about invalidity',
    layout={'width': '700px'}
)
widget

[jasongrout]

Perhaps also the sliders could inherit from this style so you could control their readout widths too.

[jeschwar]

I was just about to create an issue to request a larger readout width for the SelectionRangeSlider widget but I think this issue addresses it. Looking forward to this development on what is an amazingly useful data analysis tool!

[goatchurchprime]

For the moment it might be useful to note in the documentation the non-intuitive omission of the "readout_width" feature in the style given that there is a "description_width":
https://ipywidgets.readthedocs.io/en/latest/examples/Widget%20Styling.html

I spent ages looking for this feature (among various different margin settings), which I needed to improve the packing of these generously spaced slider widgets into my small screen to make them more useful, not knowing that this feature didn't exist.

[jasongrout]

Even better - how about we add readout_width as another style attribute? Anyone up to making a PR?