[DX] Add `#indentation` property to FAPI elements.

Description of the need

This request arose from #3292. The solution proposed there by @bugfolder alters to the core radio/checkbox widget form used for displaying the taxonomy term widget. But there was discussion about the possibility of adding a #indentation property to make this feature available to all checkbox and radio elements beyond the taxonomy term widget.

Proposed solution

Adding a #indentation property that would further theme the element using theme_indentation(). This function adds <div> elements with css class indentation to create indentation.

The new #indentation property would take an integer >= 0.

Alternatives that have been considered

The alternative is to manually add indentations to the #options values before outputting the form element. This is the current solution used in #3292.

Alternatively, it may be possible to add different css classes to each element depending on their depths. This was an earlier approach in #3292, but it was cumbersome, as it required the creation of several css classes, one for each level.

I will try to provide a PR later today.