StorageModel changes id() on the fly, so move uniqueIdOfLastMessage (read + write)
from manager to storagemodel, and call it there on demand from model.cpp

Store preSelectionMode in Pane, since Model sets it back to None after applying it,
and reapply it whenever clearing the messagelist (i.e. between deselect and select).
Also, don't disable pre-selection after the first job completes, more data could be coming in batches.

Some of this is *probably* unnecessary for simple folder-switching after the following
commit (making offline mode work more like online mode), but these code paths
(reusing a StorageModel instead of changing an existing one) are also used when
selecting multiple folders.

This commit was brought to you by a SSD disk breaking during travel, forcing me
to do the work twice :( Lesson learned, save large patches on USB key during travel.