Backend:
- replace values in constraints with Qt properties, and use properties for saving/loading
  from XML, and put XML in Constraint superclass
- monitor APG patent: US 11/994425
- actually have the speed/accuracy slider do something
- implement a playlistprovider (per Bart on merge request)

Constraints:
- fix preventduplicates: the delta functions are kinda broken
- checkpoint constraint
- last.fm or echonest similar artists constraint
- proper handling of empty fingerprints in SimilarityChain

GUI:
- "simple" presets?
- context menu for APGCategory
