Fix title generator for list items

List item labels consist of label_singular and label_plural neither of which is nullable.
* The previous version of this plugin tried to save label_singular and label_plural, and this always resulted in an error and then in a recursion.
* Additionally this trims the labels to their maximum possible length before comparing if they have been updated.
Merge pull request #105 from kehh/fixTitleGeneratorForListItems

Fix title generator for list items