Checkbox: add 'indeterminate' prop

[grasdira]

Enhancement Suggestion

I think it would be nice to add indeterminate (or isIndeterminate) prop to Checkbox component.

[tulup-conner]

I didn't even realize that's a thing. I'll convert this into an issue under Feature Request. We should definitely support it. I personally prefer prefixing booleans by is but in this case the official JS API calls it just indeterminate on DOM elements so that's probably best.

[alebo611]

Any progress on this? It seems pretty standard nowadays with an indeterminate state, for example a "select all" checkbox that controls checkboxes one level below

[rluders]

Check #871 issue. Any PR fixing it would be welcome.

[tulup-conner]

Closing in favor of #871 - I should have "converted to issue"