Tracking: Checkbox selection & indeterminate (parent) state

[TrevorBurnham]

Summary

First-class support for checkbox-driven selection, including an indeterminate state on parents whose children are partially selected (as Mantine and similar libraries expose).

Rolls up

• Indeterminate select state #312 Indeterminate select state
• Selecting parent node checkbox as intermediate #173 Selecting parent node checkbox as intermediate
• Checkbox: Treating selection differently than focus #267 Checkbox: treating selection differently than focus
• How to manage selections using checkbox? #190 How to manage selections using checkbox

Relates to the controlled-selection work in #345.