Tracking: First-class controlled-selection API

[TrevorBurnham]

Summary

Consolidating the recurring requests for a properly controlled selection model. Today selection is effectively uncontrolled: syncing it from outside (via setSelection/selectMulti + onSelect) is fragile and prone to feedback loops, and onSelect fires on programmatic changes too.

Desired outcome

• A controlled selection prop (e.g. a selected/selection map) that is the single source of truth.
• onSelect reports user-driven changes only, not changes the consumer caused.
• No infinite loops when mirroring selection into external state.

Rolls up

• How do you implement controlled select? #243 How do you implement controlled select?
• Manipulating state of selected ids from outside of the Tree component is causing infinite loops.  #176 Manipulating selected ids from outside causes infinite loops
• Do not call onSelect when selection prop changes. #189 Do not call onSelect when selection prop changes
• [HELP] Multiple selection from array of ids #221 Multiple selection from array of ids
• Selection methods double-fire onSelect when calling setSelection #332 Selection methods double-fire onSelect when calling setSelection

Closing the above against this issue once an API lands.