Added new extension to SIF/Item

ItemArrayData:
Extends items to have an array of data instead of just one cell.
Includes functions for setting, appending and getting array data.
Added new callback to SIF/Item

OnItemTypeDefined(ItemType:itemtype)

Called:
After an item type is defined.
Parameters:
<itemtype> (int, ItemType)
The ID handle of the newly defined item type.
Returns:
(nothing)