The filter-model was catching dives-added / dives-deleted signals
from the models to keep track of the number of shown dives.
To simplify the data flow, do this directly in the undo-command.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
In the future we might want to use undo-commands for mobile as
well (even if not implementing undo).
Therefore, move the undo-command source from desktop-widgets
to their own commands top-level folder.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>