search-tool: Various improvements:
- Support searching by genre again (Closes: #158)
- Prioritize search results with more entries (Closes: #396)
- Limit displayed search results to 20
- Add a label indicating when search results have been hidden
- Remove useless "Unknown Artist" and "Unknown Album"
- Remove redundant results for artists with only one song
- Perform search immediately if Enter is pressed before timeout fires
- Fix unstable sort order for songs with the same title