implement fix for nested FilterComparers + other fixes

[geo-martino]

Changed

• :py:class:.ItemSorter now accepts shuffle_weight between -1 and 1 instead of 0 and 1.
  This parameter's logic has not yet been implemented so no changes to functionality have been made yet.
• Move :py:meth:.get_filepaths from :py:class:.LocalTrack to super class :py:class:.File

Fixed

• :py:meth:.PrettyPrinter.json and :py:meth:.PrettyPrinter._to_str converts attribute keys to string
  to ensure safe json/str/repr output
• :py:class:.FilterMatcher and :py:class:.FilterComparers now correctly import conditions from XML playlist files.
  Previously, these filters could not import nested match conditions from files.
  Changes to logic also made to :py:meth:.Comparer.from_xml to accommodate.
• :py:class:.XMLLibraryParser now handles empty arrays correctly. Previously would crash.
• Fixed :py:class:.Comparer dynamic process method alternate names for in_the_last and not_in_the_last