load ogg music from music/ and sounds from sound/

[kamicane]

load ogg music from music/ and sounds from sound/, for consistency and portability of data files with other source ports when loading files always look for a lowercase version for consistency also cache the ogg versions of sounds when found

[vanfanel]

@jonof Please merge this one, it makes JFDuke3D usability MUCH better on the music front. Inconsistent OGG filenames and having to store them outside the same directory of the executable are big handicaps, and this fixes them.