Fixed the main CMakeLists.txt because it included a directory excluded by .gitignore.
Started working with the game_engine library.
Added an utility template for simple subsystems.
Improved the storage of entities in subsystems. Now they are sorted for faster removal, but not after every addition because that would be inefficient. Instead, we compare the percentage of unsorted entities against a limit and if it greater they are sorted.
Fixed a big mistake with the previous commit. I hadn't defined the limit and it's type was incorrect.