[Feat] Add indeterminate checkbox for checkbox group

[m-m-mic]

When we have a parent checkbox with children, the parent checkbox should display as indeterminate if not all children have the same state.

Example from IntelliJ:

[m-m-mic]

Implemented on branch, currently there's a bug when all checkboxes are selected and you deselect two and then click the parent checkbox again, one of the deselected checkboxes doesn't change appearance (but its value still gets properly added to the values array).

[m-m-mic]

Implemented on branch, currently there's a bug when all checkboxes are selected and you deselect two and then click the parent checkbox again, one of the deselected checkboxes doesn't change appearance (but its value still gets properly added to the values array).

Resolved issue, branch should now be ready for review.