Constify argument of playlist_open() and change_current_mood().

The only reason these arguments are not const is that we create an
osl object out of it, which contains a non-constant data pointer.

The osl library functions we call here will not touch this memory,
so it's safe to let both functions take a const pointer and cast it at
initialization of the non-constant ->data pointer of struct osl object.

The single caller of each function is activate_mood_or_playlist()
whose argument can now also me made to point to constant memory.

Finally, the arg pointer of com_select_callback() is passed to
activate_mood_or_playlist(), so the variable can be of type const
char * as well.