Spritesheet config for units and 2 minor code fixes

Added a function based on the portrait on to the unit_types.
Fixed the spacing on one line in unit_types.cpp and added the
missing type of SPRITESHEET to the enum in images.hpp that, didn't
get commited when images.cpp got commited and depended on that change.
Added a comment to draw attention to an issue
Merge pull request #227 from Aishiko/spritesheet

will still not compile however there are notes in the code about whats happening.