EntityX Integration

- Added CMake configuration for automatically downloading and building
the entityx library.
- Added CMake script to (hopefully) automatically setup the filters in
Visual Studio projects.
- Added Transform component (though I probably didn't do it right).
- Reworked header setup a bit. I should be able to further unfuck things
now that entityx is clearning the issues I was having with my component
system.
- Gutted Scene's existing functionality and made it the new EntityX
manager.