Add property callback function performImport.

[Tobias Schlauch]

Relates to: datafinder.script_api.item.item_support.performImport

Add a parameter which allows specification of a call back function which is called on every item which should be imported with following signature:

@param itemDescription: Currently traversed item.
@type itemDescription: ItemDescription
@return: Properties which should be set for the imported item.
@rtype: C{dict} of C{unicode}, C{object}

This would simplify imports from the local file system to the shared repository as you can exactly determine the properties (e.g. data store, data type) of the imported item.