Animation performance fixes

Couple of fixes related to animation performance (not sure why the old animation fixes also show up?) based on some profiling;

1) Remove findslot from skeleton and findslotindex from skeletondata, as these get called a lot during animation:apply. Trading off a little bit of memory to make a look-up table for them to save on cycles spent looping through the lists. Maybe should do the same for the rest of them, but these were by far the most called ones.
2) Only update attachments in slots if they have changed (or are empty)

br,
  Matias