Playlists can now reference songs in other libraries using relative paths.
Previously, relative paths like '../Songs/abc.mp3' would not resolve correctly
when pointing to files in a different library than the playlist file.
The fix resolves relative paths to absolute paths first, then checks which
library they belong to using the library regex. This allows playlists to
reference files across library boundaries while maintaining backward
compatibility with existing single-library relative paths.
Fixes#4617
6a3fef8e3b