Finally commit Metadata on Buffer!

Finally commit Metadata on Buffer!
buffering.c and buffering.h implement the new buffering API. playback.c is translated to that API. For more information about the whole concept, see http://www.rockbox.org/wiki/MetadataOnBuffer.
There should be no major visible changes, but most existing bugs remain (though fixing them should be easier now that playback.c is a bit less complex) and there probably will be new ones. Please report any problem!
Next step is to adapt cuesheet support, which is partly disabled here, and of course fix as much bugs as possible.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@15306 a1c6a512-1295-4272-9138-f99709370657