Introduces empty group filtering to ship browser stage 1
Added caching to empty group filtering

Stage 2 now sums total fittings in category and sets the cache to True if t_fits > 0
Stage 3 sets the cache to True if number of fits for ship > 0. This takes care of creating new fits

If fit is deleted, and there are no other fits for the ship, it reverts back to stage 2, which should set the correct value in the cache
Merge pull request #149 from blitzmann/issue-134

Introduces empty group filtering to ship browser: stage 1