MDL-24419 (1): Core restore steps should have after_restore function

At present core restore steps cannot have an after_restore function,
even though plugin restore steps can. Technically it would be possible
to just override the launch_after_restore_methods function but this
is not very neat. Instead, I added code to call after_restore function
(exactly the same way after_execute works).

backup/util/plan/restore_structure_step.class.php

@@ -395,6 +395,8 @@ public function launch_after_restore_methods() {
                 $pobject->launch_after_restore_methods();
             }
         }
+        // Finally execute own (restore_structure_step) after_restore method
+        $this->after_restore();
     }
 
     /**
@@ -408,6 +410,16 @@ protected function after_execute() {
         // do nothing by default
     }
 
+    /**
+     * This method will be executed after the rest of the restore has been processed.
+     *
+     * Use if you need to update IDs based on things which are restored after this
+     * step has completed.
+     */
+    protected function after_restore() {
+        // do nothing by default
+    }
+
     /**
      * Prepare the pathelements for processing, looking for duplicates, applying
      * processing objects and other adjustments