Fragile Glass and Thin Ice

* Now suggests all behaviour types when expecting one in a command

* When viewing block/tile entity behaviours, now shows their list item position

* When removing a behaviour from a list of length 1, removes the key instead of leaving an empty list

* The Modify Command has been split into Add (which inserts items at a given position in the list) and Modify (which changes the item at a given position). For Entities, Add and Modify work the same as the original Modify (replace the single item).