This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Replaces GLib's GError.

Migrate all callers to use Path directly, instead of doing the
conversion in each caller.

These fixes were mostly generated with `codespell' [0] and manually
reviewed.

[0] http://git.profusion.mobi/cgit.cgi/lucas/codespell/

sticker_delete_value() deletes only one value in a sticker, while
the old function sticker_delete() deletes all values.

sticker_find() finds stickers with the specified name.

sticker_list_values() is only used internally in sticker.c.  Remove
sticker_song_list_values() completely, it is superseded by
sticker_song_get().