Move includes from .hpp files to .cpp files

[JenniferPylko]

If more things are forward-declared instead of included in header files, and are then included in the cpp file (exception is API.hpp), then this will make users of MINX get into a good habit of including all the headers they need and not just including one knowing that others will be indirectly included. This may not be easy/practical with some classes in the standard library or some GL header files that make heavy use of typedef.