Cover art support

- implement database scheme
- search for embedded cover
- search for cover in track directory
- save covers in disk cache
- search for covers in disk cache
- minimal ui (display covers in lower left corner)
- handle existing users
- LRU cache
a bit style nitpicking. Can you please change this to our default style of

```C++
if (!query.exec()) {
  LOG_FAILED_QUERY(query);
  return coverArtInfo();
}

// handle query results
```