Fix handling of tag changes
This PR contains a series of changes to tag handling that make the itemsTagsChanged notification actually usable.
The last change requires a change to the Akonadi protocol and the ChangeRecorder journal format, but the format is already versioned, so it should not cause any problems.
With this set, the use-case of a deleted tag getting untagged from all Items and such change propagating all the way to resources works correctly (as long as resources handle it properly).
Edited by Daniel Vrátil