- don't try to encode anything, but pass the filename into the argv for the
program to open (before we, used g_spawn_command_line_async, and there we
need to escape things for the command line. Now we use g_spawn_command, so
we can pass argv as such.
- add a GError** arg to mu_util_play, and update all callers
- cleanup the mu-cmd-extract code a bit