Change the distribution of the default skin

[fmang]

Right now, the default skin is downloaded when make is run, unless it is already present.

Distribution tarballs are shipped with the samples for the regular user. That default skin accounts for a very large part of the weight of the tarball.

The alternative is to distribute 2 tarballs: one with the program, and one with the default skin (since it never changes), and make it easy for the packagers to integrate the skin if needed.

A rule make install-default-skin could provide a convenient way for the user to install the skin, or maybe a more complete oshu-skin command. If the skin is named .osk it makes it obvious the user is free to use another skin.

Now, it would be nice to include at least one default sample to ship with the program tarball so that the game remains playable without any skin. The -GN sample is a really nice candidate here, and solves #58.

Thank you @lanodan for your advice :)

Proposal for the directory structure:

/usr/share/oshu/skins/
    current (or default, as in OSHU_SKIN=default) -> gn
    gn/ (or min, default, standard, oshu) 
        normal-hitnormal.wav
        soft-hitnormal.wav -> normal-hitnormal.wav
        drum-hitnormal.wav -> normal-hitnormal.wav
    osu/
        normal-hitnormal.wav
        soft-hitnormal.wav
        ...

[fmang]

I managed to make all these things configurable at build time with CMake. The README contains the new details. In short: -DOSHU_DEFAULT_SKIN=osu makes cmake download and install the osu skin on top of the minimal one, and creates a default symlink :)

Next is #64.