Return reference from Get_New_Partition() (#757671)

Return newly constructed partition object by reference rather than by
copy from the Copy, Resize/Move and New dialog classes.  This is another
case of stopping copying partition objects in preparation for using
polymorphic Partition objects.  In C++ polymorphism has to use pass by
pointer and reference and not pass by value, copying, to avoid object
slicing.

The returned reference to the partition is only valid until the dialog
object containing the new_partition member is destroyed.  This is okay
because in all three cases the returned referenced partition is copied
into a context with new lifetime expectations before the dialog object
is destroyed.

Case 1: GParted_Core::activate_paste()
    Referenced new_partition is copied in the OperationCopy constructor
    before the dialog object goes out of scope.

    Operation * operation = new OperationCopy( ...,
                                               dialog.Get_New_Partition( ... ),
                                               ... );

Case 2: GParted_Core::activate_new()
    Referenced new_partition is copied in the OperationCreate
    constructor before the dialog object goes out of scope.

    Operation * operation = new OperationCreate( ...,
                                                 dialog.Get_New_Partition( ... ) );

Case 3: GParted_Core::activate_resize()
    Temporary partition object is copied from the referenced
    new_partition before the dialog object goes out of scope.

    Partition part_temp = dialog.Get_New_Partition( ... );

Bug 757671 - Rework Dialog_Partition_New::Get_New_Partition() a bit