OTHER: Don't convert xmmsv_t to GArray for playlist cache in cli.
OTHER: Move command_context_t related code to command_utils.c.

Should probably rename command_utils.c as well, but that's for later.
OTHER: Intersect query with playlist where applicable.
OTHER: Remove typedef of GOptionEntry in cli.
OTHER: Move command_context_t typedef to command_utils.h.
OTHER: Remove unused include in matching_browse.c.