KolabResource: We no longer require the timestampattribute
IMAP-Resource: Remove all uses of the timestamp attribute.

Updating this during every metadataretrievel resulted in a lot of updates,
in i.e. kmail, that currently are very expensive (due to the favoritesproxymodel
issue). Since it was only used in one place to determine wether the collection
is new, we can drop it entierly. I removed the knownMailboxes hack with it,
since we no longer use it anyways.
add a relationsRetrieved method
refactor out tag extraction and add relation extraction
Merge remote-tracking branch 'remotes/kolab/feature/remoterelationsyc' into kolab/integration/4.13.0
split retrieval of tags and relations
Merge remote-tracking branch 'kolab/feature/remoterelationsyc' into kolab/integration/4.13.0
KolabResource: Extract relation members just like tag members.

We cannot simply store the item id.
KolabResource: Not finding a configuration folder is not an error.