#50 warn user about updating wrong CD

[Helge Hielscher]

steps to reproduce:
1. mount CD1 in prokyon3
2. append CD1 to database
3. umount CD1, change CD1 to CD2, mount CD2 with other programms
4. update CD

result: prokyon thinks CD1 is still inserted and updates CD1 instead of appending/upgrading CD2

expected result: a dialog, like "do you want to add another disk as 'name of CD1'"