QOrganizer: Fix handling of item IDs
Created by: mardy
This commit heavily reworks the way that item IDs are passed around. First we add a couple of static functions to QOrganizerEdsEngine, to translate from EDS coordinates, which consiste of a source ID and an item ID, to QtOrganizer coordinates, which consist of a single QOrganizerItemId (inside which we encode both athe EDS source and item ID).
Then, we stop using a QString to store the collection ID, but either use the proper QOrganizerCollectionId class or, wherever possible, just the EDS source ID (which corresponds to QOrganizerCollectionId::localId()).