Stop returning vector of partitions from Dialog_Rescue_Data class (#759726)

get_partitions() method was returning a vector of partitions.  However
the calling code only needed to know whether any partitions were found
or not.  Replace with found_partitions() method reporting the needed
boolean.

Now use of std::vector<Partition> partitions is hidden within the
Dialog_Rescue_Data class implementation.

Bug 759726 - Implement Partition object polymorphism