Begins adding a new cache item.
Unique item identifier.
Optional region (sub-directory) name.
Returns the path to the physical file that will hold the data for the item on disk.
This method calls BeginAddExternalResource and will be removed from the next version of LEADTOOLS.
LEADTOOLS Document classes uses this method to quickly add items. It speeds up access and updating the data if the value contain large amount of data such as an image file.
After this method returns, EndAdd must be called to either commit this item into the cache or discard it.