Commit 0aef04352a3210a6024f860758af466ea774dd5e made it impossible
to move new dive sites on the map. When entering dive-site-edit mode,
the filter would be instructed to show only dives of the corresponding
dive site [which didn't yet exist] and therefore all dive sites
on the map were deselected.
Fix this by explicitly centering on the dive site to be edited in
MapWidgetHelper::enterEditMode().
Fixes#1809
Reported-by: Jan Mulder <jlmulder@xs4all.nl>
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>