Tracking: Search introspection & callbacks (match count, no-results, displayed data)

[TrevorBurnham]

Summary

Consumers need to observe the result of a search: how many nodes matched, whether there were zero results, and what the resulting/visible tree is — to render "no results" messages, counts, etc.

Rolls up

• Feature request: expose how many matches there are when filtered #112 Expose how many matches there are when filtered
• Is it possible to intercept after search is complete and get the displayed data  #286 Intercept after search completes and get the displayed data
• No search results message #256 No search results message
• visibleNodes does't return always proper values  #157 visibleNodes does not always return proper values during search
• How to show all children if a parent matched. #177 Show all children if a parent matched