Cleanup: use getDiveSelection() for shifting time

There are two cases where dive-times are shifted: in an explicit
dialog and when editing the date/time of a dive.

In each of these cases, the selected dives were collected manually.
Instead use the getDiveSelection() function. Since this returns
a std::vector, change the argument of Command::ShiftTime() to
such a std::vector.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

commands/command.cpp

@@ -24,7 +24,7 @@ void deleteDive(const QVector<struct dive*> &divesToDelete)
 	execute(new DeleteDive(divesToDelete));
 }
 
-void shiftTime(const QVector<dive *> &changedDives, int amount)
+void shiftTime(const std::vector<dive *> &changedDives, int amount)
 {
 	execute(new ShiftTime(changedDives, amount));
 }