Add api call to set the player master volume

Use xmp_set_player() with XMP_PLAYER_VOLUME to set the player master
volume (ranges from 0 to 100).

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[xmdp] Remove xmdp sources

Move xmdp to its own repository.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>