* Attempt to improve readability by using focused, coherent helper classes.
* Replace ListBase with blender::Vector, which is more efficient and has a
better API.
* Split user reporting from error checking.
* Use namespace (as we usually do for C++ code).
* Remove unused headers
It seems generally preferred to have new files be created with C++.
The only reason I didn't do that when I initially created the files is that I
was unsure about some C-API aspect.
Also includes some minor C++ related cleanup (nullptr instead of NULL, remove
redundant `struct` keyword).
