Restructure the source tree

[fmang]

Because I'm dealing with cmake #59, and because the number of source file has gotten important, maybe it's time to restructure the layout of the project.

An important aspect of the future release is that oshu will ship with a few extra executables that share the same modules as the game, and especially the beatmap module.

Here's the structure I have in mind:

include/
    external .h headers
lib/
    internal .h headers
    .cc sources
src/
    oshu.cc
    oshu-library.cc
    oshu-skin.cc
test/
share/
    man/
    skins/
    desktop/

This structure is heavily inspired from LLVM, with the exception of share which I made up from the FHS, and that oshu's been using so far.

The lib/include directories don't mean that liboshu will be installed as a dynamic library, but it will exist at least as the internal library shared between the oshu executables.

[fmang]

Done.