Rename DomainSlider to DomainWidget

[axelboc]

First step in addressing #1382 and #1377. This is a breaking change, obviously. The idea is to then rename ScaledSlider to DomainSlider and export it.

The end goal is to have the following components exported:

• DomainWidget: the whole toolbar control that allows choosing a custom domain (i.e. slider + tooltip);
• DomainSlider: the actual slider;
• DomainControls: the content of the tooltip (bounds inputs + auto-scale buttons).

The tooltip behaviour will remain the responsibility of DomainWidget.

I haven't renamed the CSS variables for now -- I'll do it after I rename/create the other components.

[axelboc]

I've created a branch called domain-widget into which I'll merge this and the next PRs. I won't merge domain-widget into main until we're ready for another major release.

[loichuder]

We already have #1439 which is technically a breaking change so you can merge to main if you wish.