make sure screen numbers are consecutive

since 5.27 all screen numbers in Containment::screen() are guaranteed to be consecutive (0 is primary, 1 is the secondary and so on) when unserializing from the screen mapping config make sure all the saved screen numbers are consecutive, and if they aren't remap them to make it so. This gives a migration path from old plasma versions and adds anyways an important consistency check in the config on normal runtime