undo: make adding of pictures undoable

This one is a bit hairy, because two things might happen if the
picture has a geo location:
- A dive gets a newly generated dive site set.
- The dive site of a dive is edited.
Therefore the undo command has to store keep track of that.
Oh my.

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

commands/command.h

@@ -131,6 +131,7 @@ struct PictureListForAddition {
 };
 void setPictureOffset(dive *d, const QString &filename, offset_t offset);
 void removePictures(const std::vector<PictureListForDeletion> &pictures);
+void addPictures(const std::vector<PictureListForAddition> &pictures);
 
 } // namespace Command