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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.
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