Set a name for each individual page that sticks with it.

The name associated with each page was "Page i", where i was the index in the list. This meant that when deleting or reordering the scans, the names would not stay with the scan. This also makes it possible to extend so that the user could name each individual scan, and the names could then be used as the filenames when saving them (if saved as a non-PDF).

By having a name that is associated with the actual scan, it is easier to identify the actual scan. This makes it easier to track when scanning the same image repeatedly, such as during testing with the sane "test" backend.

I also had a feeling that the reordering buttons were not working properly, and by keeping having the name move with the page, it was trivial to see that.