Rename Dialog_Base_Partition member to new_partition

The member variable was named selected_partition.  It is assigned from
Win_GParted::selected_partition_ptr (which is a pointer to a const
partition object so is never updated).  This gives connotations that it
won't be modified.

However it is updated freely as the new resultant partition object is
prepared before being returned from the dialog, most notable in the
Get_New_Partition() methods.

Therefore rename from selected_partition to new_partition.