Add and use Partition::set_unpartitioned() method (#788308)

PATCHSET OVERVIEW:

When unpartitioned drive read-write support was added this commit added
a whole_device flag:
   5098744f
   Add whole_device flag to the partition object (#743181)

Using a whole_device flags now seems not the correct way to model
unpartitioned drives.  GParted models an uninitialised drive as:
    .path         = _("uninitialized")
    .type         = TYPE_UNALLOCATED
    .whole_device = true
    .filesystem   = FS_UNALLOCATED
and a whole drive file system, using ext4 for example, as:
    .path         = "/dev/sdb"
    .type         = TYPE_PRIMARY
    .whole_device = true
    .filesystem   = FS_EXT4
No partitioning changed yet the type of the partition in the model
changed between TYPE_UNALLOCATED and TYPE_PRIMARY depending on whether
the whole drive contains a recognised file system or not.

The partition object describing a file system within a LUKS encryption
mapping is another case of the model not matching reality.
    .path         = /dev/mapper/crypt_sdb1_crypt
    .type         = TYPE_PRIMARY
    .whole_device = true
    .filesystem   = FS_EXT4
There is no partition table within the encryption mapping, the file
system fills it, but GParted records it as a primary partition.

Make TYPE_UNALLOCATED and TYPE_PRIMARY be reserved for representing
unallocated space and primary partitions within a partitioned disk drive
and introduce new TYPE_UNPARTITIONED for all cases of an unpartitioned
whole disk drive.

The GParted UI does differentiate between an unallocated whole disk
device and anything else by requiring a partition table to be created
first, even if that is just the loop partition table.  That
determination can simply look for the partition object containing file
system type FS_UNALLOCATED instead.

THIS PATCH:

Create set_unpartitioned() helper method to set a partition object to
represent a whole disk drive and use everywhere such an object is
modelled.  This matches what existing methods Set_Unallocated() and
indeed Set() do for unallocated space and any type of partition
respectively.

For now the partition type is still set to either TYPE_UNALLOCATED or
TYPE_PRIMARY so the rest of the code base remains the same.
TYPE_UNPARTITIONED will be introduced later.

Bug 788308 - Remove whole_device partition flag