Match smartplaylist options with playlist plugin

### Proposed solution

Add options like `library` as possible value for the `relative_to` key in the `smartplaylist` configuration.

#### Goals

Using smartplaylist and playlist in tandem without hardcoded paths.

Start by locating the smartplaylist and playlist plugin implementations and tracing how the relative_to configuration value is handled. Confirm how the library option should interact with both plugins; done means smartplaylist and playlist can be used together without hardcoded paths.

Written by the indexing model from the issue text.