When pressing F2 in the dive list, the number can be edited.
Make this action undoable by implementing a EditNumber command.
This command is differs from the other undo commands, as not the
currently selected dives are changed. This means that the EditCommand
needs an alternative constructor taking a single dive. This constructor
was implemented in the base class so that all edit commands can now
be called with a single dive.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>