logic: add name delimiter to NameHierarchy

* NameHierarchy now looks like this: <delimiter>\tm<all the rest>
* made new baseclasses for SettingsMigration and SettingsMigrator
* Use the migration system for SqliteBookmarkStorage as well

src/lib/data/access/StorageAccessProxy.cpp

@@ -73,7 +73,7 @@ NameHierarchy StorageAccessProxy::getNameHierarchyForNodeId(Id id) const
 		return m_subject->getNameHierarchyForNodeId(id);
 	}
 
-	return NameHierarchy();
+	return NameHierarchy(NAME_DELIMITER_UNKNOWN);
 }
 
 std::vector<NameHierarchy> StorageAccessProxy::getNameHierarchiesForNodeIds(const std::vector<Id> nodeIds) const