Stop copying selected_partition back on itself in the copy dialog (#750168)

The code goes like this:

    Dialog_Partition_Copy::Get_New_Partition()
        call Dialog_Base_Partition::Get_New_Partition()
            Update this->selected_partition with results from running
            the dialog.
            return this->selected_partition by value.
        Save value back to this->selected_partition.
        Update this->selected_partition some more.
        return this->selected_partition by value.

So there is an unnecessary copy of the partition object returned from
the base class Get_New_Partition() function back to the same variable in
the derived copy class Get_New_Partition() function.

Need to keep the base class Get_New_Partition() function as derived
class Dialog_Partition_Resize_Move uses that implementation as it
doesn't override it, and it's part of the interface.

Avoid this unnecessary copy by moving base class Get_New_Partition()
code into a new private function, called prepare_new_partition(), which
doesn't return anything.  Then have Get_New_Partition() in both classes
just return the required partition object.  Like this:

    Dialog_Base_Partition::Get_New_Partition()
        call prepare_new_partition()
        return this->selected_partition by value.

    Dialog_Partition_Copy::Get_New_Partition()
        call Dialog_Base_Partition::prepare_new_partition()
        Update this->selected_partition some more.
        return this->selected_partition by value.

Bug 750168 - Reduce the amount of copying of partition objects