fix some issues when editing a topic title and then cancelling it

Meta: [Editing a topic title and canceling the edit](http://meta.discourse.org/t/editing-a-topic-title-and-canceling-the-edit/4722/)

- [x] updated the `ComboboxViewCategory` so that it uses a `oneWay` binding for the category name preventing direct update.
- [x] removed the patch done in #450 as it is no longer needed