Add partition name to Create New Partition dialog (#746214)

Add a partition name entry box to the Create New Partition dialog.  The
entry box is greyed out (not sensitive) for partition table types which
don't support partition naming.  Currently only supported for GPTs.  See
Utils::get_max_partition_name_length() for details.

There was a slightly wider gap between the file system combobox row and
the label entry row when there were only three widgets on the right hand
side of the dialog.  This has been removed now that there are four
widgets so that they are all evenly spaced and they line up with the
four widgets on the left hand side.

So far the partition name can be entered and previewed, but isn't yet
applied to the disk.

Bug 746214 - Partition naming enhancements