Adding new sorting key

1. Update SK_* items in ui/ui.h, that is add new value to the end of the list and update SK_LAST. New elements should go to the end because otherwise they will break restoring sorting keys from vifminfo files.
2. Compile errors will show arrays/switches that need an update (with -Werror, just configure as ./configure --enable-developer ...).
3. Note that order in sorting dialog differs from enumeration values and is mapped to them via indexes indirection array in modes/dialogs/sort_dialog.c.