client: speed up filter searching

For large libraries it takes a long time to populate the library index information. This is exacerbated by the fact that audio stream playback is done in the same thread as JavaScript, so slow filter queries can cause audio skips and glitches.

Ideas to speed this up include:
- [x] putting a short delay on the filter box so that we don't queue up more than 1 filter when keys are pressed while already filtering
- [x] updating DOM elements instead of deleting and recreating them
- [ ] using the audit feature of chrome to see where most time is spent
- [ ] not rebuilding the entire library index for a single change; ability to update the library index one song at a time
- [ ] when > N items are matched by a given filter query abort the query early