GUi: add save sample as raw data

src/engine/sample.h

@@ -201,6 +201,13 @@ struct DivSample {
    */
   bool save(const char* path);
 
+  /**
+   * save this sample to a file (raw).
+   * @param path a path.
+   * @return whether saving succeeded or not.
+   */
+  bool saveRaw(const char* path);
+
   /**
    * @warning DO NOT USE - internal function
    * initialize sample data.