The DiveImportedModel and DownloadThread used the same table
of dives and dive sites. This made it very hard to keep the
model consistent: Every modification of the download thread
would make the model inconsistent and could lead to memory
corruption owing to dangling pointers.
Therefore, keep a copy in the model. When updating the model,
use move-semantics, i.e. move the data and reset the tables
of the thread to zero elements.
Since the DiveImportedModel and the DownloadThread are very
tightly integrated, remove the accessor-functions of the
dive and dive-site tables. They fulfilled no purpose
whatsoever as they gave the same access-rights as a public
field.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>