disableParentChange option added.

To be able to disable changing parentship but allow reordering in current parent.
I use it for a multi level menu to be able to order the pages, where pages define parentship and a menu (re)defines the sort order (no parentship needs to be stored in the menu_pages table yet reordering is fully possible)
What do you think of it? (maybe it could be done easier / cleaner, but I haven't had time to find out yet)