core: Letting filter() accept lists
Adding a 'to list' conversion and correcting horrible mistake in comparison (put it reverse)
Adding test case
tests: Fixing self confusion mistake
docs: Documenting tracklist's new filter() feature
core: Changing input to accept also sets, as might be also used.
core: filter() to accept also tuples
Merge branch 'develop' into feature/filter_improvement
core: Change tracklist.{filter,remove} usage

The criterias are now a mapping between field names and one or more values.
This aligns tracklist.{filter,remove} with the API of
library.{find_exact,search}, and allows for e.g. batch removals.

An exception is raised immediately if the API is used in the old way to ease
migration and debugging.
Merge pull request #574 from jodal/feature/filter_improvement

tracklist.{filter,remove} with multiple criteria values