Use pointers to Partitions in PartitionVector class (#759726)

The PartitionVector class is now internally using pointers to Partition
objects and taking on management of their lifetimes.  It therefore has
to implement the Big 3: destructor, copy constructor and copy assignment
operator [1][2].  This is because the implicitly-defined copy
constructor and assignment operator perform memberwise "shallow copying"
and the destructor does nothing.  This not correct for classes which
contain non-class types such as raw pointers.

The semantics of the interface still copies each Partition object into
the PartitionVector when they are added with push_back() and insert().

Note that a PartitionVector object is explicitly copy assigned in
Win_GParted::Refresh_Visual().  They are also implicitly copied when
(1) the implementing vector is resized larger to allow it to hold more
pointers to Partition objects than it previously had capacity for; and
(2) a Partition object is copied including the logicals PartitionVector
member.

[1] The rule of three/five/zero
    http://en.cppreference.com/w/cpp/language/rule_of_three
[2] Rule of Three
    https://en.wikipedia.org/wiki/Rule_of_three_%28C%2B%2B_programming%29

Bug 759726 - Implement Partition object polymorphism