dcc redesign: accumulated bugfixes

[KoolADE85]

This PR addresses issues/bugs reported so far in the existing redesigned components:

• fix bug with step in sliders and add relevant tests
• add localizations prop to dropdowns to allow customization of hard-coded text
• improve display of very long (wide) dropdown options
• improve accessibility of dropdown & option lists (screen reader improvements and fix labels' htmlFor prop not working)
• add visual focus state to inputs
• add CSS variables for shadow colours
• fix className in slider so that it is applied at the top level of the component.

To see the fixes one-by-one, just go through the commits.

[gvwilson]

@T4rk1n @AnnMarieW if you have time to do a review, it would be very welcome - thanks

[AnnMarieW]

This is great! Just a couple comments:

• Good that the focused state style was added on the Inputs, but can it be more subtle? - like the focus of the search box in the Dropdown?

• Can a disabled state style also be added? In previous version the background was a gray color. Or for consistency, it could have the same cursor style as when the Dropdown is disabled.

Input:

Dropdown

[AnnMarieW]

Also, any other comments on the issues raised in #3449?

[KoolADE85]

This PR addresses most of what's in #3449, except for the cross-component consistency portion. I'd like to tackle that portion as a separate PR!

[T4rk1n]

Looks good, just the docstring for the new prop should be changed.