Various code improvements:

- Index now runs constructors rather than filling objects with zeroes.
- Index now supports in-place object construction.
- Various copy constructors now return a proper reference.
- Many malloc/free or new/delete patterns were migrated to containers.
- strcmp_safe() and strcmp_nocase() gained an optional length argument.
- Miscellaneous unnecessary GLib usage was eliminated.
- Prototypes for aud_eq_get/set_bands() were clarified.
- PLAYLIST_UPDATE_ defines are now (void *) to avoid casts.