gpt: Fixed off by one error.

* The backup header would have been written one block behind the actual
  last block. Well, at least it would have tried to do so :-)
DriveSetup: Added column for the partition type.

* This implements ticket #4235.
BPartition::CanInitialize() now returns false if the size is 0.

* This should fix #9405; I cannot test it right now due to a missing
  CD-ROM.
Minor cleanup.
DriveSetup: Removed (some) knowledge about initialization.

* Instead of guessing whether or not to show the InitParamsPanel, the panel
  will now no longer show itself if the disk system does not have an editor,
  and silently succeed.
* This also fixed a potential crash if the editor could not be created for
  some reason.
* Minor cleanup.