Catsop widget: list conflict slices

List conflict slices in the slices table and highlight the conflict
slice mask when hovering over it in the widget table. Note that slice
loading is now very slow, because while prefetching the conflict slice
sets in django caches them correctly, each conflict set lookup during
serialization still generates a query that is logged and printed by the
debug server.
Catsop widget: fix slice mask tiling when zoomed
Catsop widget: indicate slices in solution