make ItemModelBase._storeData easier to override

[jonathanj]

Even though ItemModelBase._storeData is designate a private method (perhaps something that change), in my experience in often needs to be overridden instead of its related method, ItemModelBase.storeData in order to maintain the correct SQL transaction behavior.

Overriding this method is unnecessarily quirky, however, because ItemModelBase.storeData passes its data as an actual Python dictionary as opposed to unpacked keyword arguments a la standard Python convention.

It is very helpful in the common (at least for me) case of slightly tweaking the behavior of ItemModelBase._storeData to be able to "peel off" some of the keyword arguments passed in from the form invocation and manipulate them as needed before issuing a cooperative super() call on _storeData.