[ Feature Request ]: Add "liked" songs and play count

### Proposed solution
Add 2 columns in the database:
* liked (bool)
* play_count (int)

#### Goals
integrate with media players so you can keep track of how many times a file was played so the media player can integrate with the database and not keep track of it separately

The issue names no files or tests. Start by locating beets' database schema and the entry points used by media-player integrations, then inspect how existing fields are stored and updated. Done means persistent liked and play_count fields are available for media-player integration and their behavior is covered by the relevant tests.

Written by the indexing model from the issue text.