Dehardcode Base_animation and improve Collada support

The original post and MR idea: Move base_anim -files to settings instead of being hardcoded. This is required for later work to use other animation formats. Very briefly tested, the strings for file paths use both / and \ in a similar fashion they were found in OpenMW's codebase. This should be properly reviewed before merging, and tested thoroughly before a release, because it's potentially game-breaking.

Edit: See the discussion for other details.