Add PluginInterface::GetHeaderVersion()

include/loot/plugin_interface.h

@@ -45,6 +45,14 @@ public:
    */
   virtual std::string GetName() const = 0;
 
+  /**
+   * Get the value of the version field in the HEDR subrecord of the plugin's
+   * TES4 record.
+   * @return The value of the version field, or NaN if the field could not be
+   *         found.
+   */
+  virtual float GetHeaderVersion() const = 0;
+
   /**
    * Get the plugin's version number from its description field.
    *