add method restoreFinished and reimplement it in SketchObject

src/Base/Persistence.cpp

@@ -129,4 +129,5 @@ void Persistence::restoreFromStream(std::istream& stream)
     reader.readElement("Content");
     Restore(reader);
     reader.readFiles(zipstream);
+    restoreFinished();
 }

src/Base/Persistence.h

@@ -154,6 +154,14 @@ public:
 
     //restore the binary persistence data from a stream. Must have the format used by dumpToStream
     void restoreFromStream(std::istream& stream);
+
+private:
+    /** This method is used at the end of restoreFromStream()
+     * after all data files have been read in.
+     * A subclass can set up some internals. The default
+     * implementation does nothing.
+     */
+    virtual void restoreFinished() {}
 };
 
 } //namespace Base