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
- last.fm or echonest similar artists constraint
- playlist length (ie, # of tracks) constraint

GUI:
- context menu for APGCategory