Use `widgets.TextBox` as text readout for sliders

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### Problem
When using a slider there it is not easy to select a precise value when using the mouse. You can do this with `slider.set_val` but this is not always convenient.

### Proposed Solution

Use `widgets.TextBox` to display the slider readout. Then hook it up so that a user can enter values into the textbox and on submit the slider value will change.

To do this fully properly I think two additional things would need to happen:
1. Implement numeric only TextBox
2. if user clicks off of TextBox after typing then return the textbox to the current value of the slider.

### Additional context and prior art
Inspired by examples here: https://www.smashingmagazine.com/2017/07/designing-perfect-slider/

Start by reading the existing slider and widgets.TextBox APIs, including slider.set_val, to understand how values and submissions are handled. Done means the slider displays its value in a TextBox, accepts submitted values, supports numeric-only input, and restores the current slider value when focus is lost after editing.

Written by the indexing model from the issue text.