Do orphan block checking differently... (#655)

... Don't have special knowledge of "blockid" in ProjectSerializer, which should
be very low-level.

Instead, we can deserialize the project first, and use the block ids collected
by the sample block factory since f137a1e.

src/SampleBlock.h

@@ -13,6 +13,7 @@ SampleBlock.h
 
 #include <functional>
 #include <memory>
+#include <unordered_set>
 
 class AudacityProject;
 class ProjectFileIO;
@@ -133,6 +134,10 @@ public:
       sampleFormat srcformat,
       const wxChar **attrs);
 
+   using SampleBlockIDs = std::unordered_set<SampleBlockID>;
+   /*! @return ids of all sample blocks created by this factory and still extant */
+   virtual SampleBlockIDs GetActiveBlockIDs() = 0;
+
 protected:
    // The override should throw more informative exceptions on error than the
    // default InconsistencyException thrown by Create