Javascript live-filter functionality for widget results (for large taxonomies)

Just ran into this again and still this would be super helpful.

I also needed this for a project, and created a basic version based on the D7 patch. Still needs some work, but might be worth a quick review about the approach.

I've pushed some updates to the issue fork:

• switched to using the once() method provided by core
• added the search attribute to the config schema
• some general cleanup
• added a further option to preserve the children of matched terms

The last point is a requirement that has cropped up in a project we're currently working on. Basically you have a tree where for example the first level is cities and the second level is zip codes associated with these cities. Previously if you were to search for a city name, the zip codes would disappear. If you set the new config value to TRUE, the (non-matching) children of matching terms will also remain visible.