Mike Fleetwood authored Aug 21, 2016 and Curtis Gedak committed Jan 14, 2017

The Operation class already provided find_index_extended() method and
 was used in the Operation and derived classes where required.  It
returns the index to the extended partition in the PartitionVector
object, or -1 when no extended partition exists.

There were several cases of the same functionality being open coded in
GParted_Core and Win_GParted.  Therefore move the implementation to
find_extended_partition() in PartitionVector compilation unit and use
this implementation everywhere.