Add player states

The player can be in one of the following states:

XMP_STATE_UNLOADED: after context creation or module release
XMP_STATE_LOADED  : when a module is loaded but not playing
XMP_STATE_PLAYING : when a module is loaded and playing

Tracking the player state also allows us to proper end the player and
release the module if xmp_free_context() is called during replay, (or
end the player if xmp_release_module() is called).

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