Cleanup, fixes, and more.

BlueMoon has been converted from a visual novel game engine to a
general-purpose 2D game framework that intends to provide the basics
needed to make a 2D game with acceleration from the 3D hardware.

* Remove DSFML as a submodule, instead we now use dub to handle deps and
building.
* Update license info.
* Remove old build script.
* Restructure source directory to make use of the package.d files.
* Refactor code to comply with preferred D code style.
* Remove unnecessary files and merge utils in to the file/s where
they're needed.
* Rename some files to have less verbose module names.