COLLADA: split skeleton definitions into moddable files

[Philip]

The skeleton definitions should be located in mods/*/art/skeletons/*.xml, and all the files matching that pattern should be loaded. That would allow modders to add their own skeleton definitions, just by creating a new file (rather than overwriting the existing one). It would also reduce the number of animations that get cache-invalidated when a single skeleton definition is tweaked.

[anonymous]

Milestone ASAP deleted