Bugfix #5027: Deleting Shape key sometimes caused corruption to Ipo curves.
sort_key() was moving key but not resorting adrcodes for keys and Ipo curves.
Also changed behavior of sort_key() so that it only resorts one key; all the
places which currently call it only do so after changing one key (the
previous code was moving the key one location at a time through the keyblock
list).