Change copied_partition into a pointer (#759726)

Change Win_GParted::copied_partition from Partition object which is
copied by value into a pointer to a Partition object object which is
allocated, copy constructed and deleted.  Required as part of the
polymorphic implementation of Partitions.

As before when managing the lifetime of pointers to objects in a class
the Big 3 of destructor, copy constructor and copy assignment operator
need to be considered.  A destructor is added to finally delete
copied_partition.  A single Win_GParted object is only ever created and
destroyed in main().  The class is never copy constructed or copy
assigned.  Make the compiler enforce this with private declarations and
no implementations.

Bug 759726 - Implement Partition object polymorphism