Replace all Partition object copy assignment (#759726)

Copy assignment of Partition objects is now only performed in a few
places in the Operation and OperationResizeMove classes when updating
the displayed PartitionVector.  (From Refresh_Visual() when each
operation is visually applied to the display_partitions vector; the
new_partition from the operation is copy assigned over the top of the
relevant existing partition in the display_partitions vector).

In general polymorphic copy assignment is complicated [1], and is now
unnecessary given the above limited use.  All that is needed is a way to
polymorphically replace one Partition object with another in a
PartitionVector.

First, prevent further use of Partition object copy assignment by
providing a private declaration and no implementation, so the compiler
enforces this.  Second implement and use PartitionVector method
replace_at() which replaces a pointer to one Partition object with
another at the specified index in the PartitionVector.

[1] The Assignment Operator Revisited
    [Section:] Virtual assignment
    http://icu-project.org/docs/papers/cpp_report/the_assignment_operator_revisited.html

Bug 759726 - Implement Partition object polymorphism