Consolidate down to a single insert_unallocated() implementation (#759726)

GParted_Core and Operation classes both have an insert_unallocated()
method which do the same thing with very nearly identical code.  Both
methods insert unallocated partitions into the vector of partitions
within the specified range of sectors to fill in any gaps larger than
1 MiB.  The only difference was how the two methods got the device path;
the GParted_Core class method got it via a parameter and the Operation
class method got it by calling get_path() on its device member variable.
The GParted_Core insert_unallocated() method gets called during device
scanning and the Operation one gets called when constructing the visual
for a pending operation.

Consolidate down to a single insert_unallocated() implementation by
making the Operation class method call the GParted_Core class method.
Make the GParted_Core class method static and public so that it can be
called using the class name from outside the class.

Bug 759726 - Implement Partition object polymorphism