Use platform prefix for files specific to that platform AND that have no
credible alternative API on that platform.
The exception is WinMM, which will be replaced before 3.2.
The public, platform, native, event and utility functions are already
documented in-source. Having duplicate documentation inevitably means
having them contradict one another. Furthermore, most of the function
descriptions simply repeated the name of the function.
Renamed _glfwLibrary to _glfw and made all substructs lower-case, making
global variable names easier to read and type. Partially inspired by the
internal naming conventions of glwt.
