QgsDataItem::providerKey has different semantic in subclasses

[elpaso]

QgsDataItem::providerKey() is supposed to return the QgsDataItemProvider name (key) but all layer items are (correctly, because that value is required for drag and drop mime types) initiated with the name of the QgsDataProvider associated with the layer and hence they return the QgsDataProvider key and not the QgsDataItemProvider key.

I'm proposing to:

• deprecate and rename QgsDataItem::providerKey to QgsDataItem::dataItemProviderKey (always valued, make the dataItemProviderKey a mandatory argument in QgsDataItem ctor)
• add QgsDataItem::dataProviderKey (possibly blank because not all data items have a corresponding QgsDataProvider)
• rename and add setters accordingly

[m-kuhn]

This can be done outside a 4.0 change.

[elpaso]

Not with the ctor changes.

[m-kuhn]

Agreed. In this case only 90% of this change can be done outside a 4.0 change.