Improve synchronization for TextGridObjects [was #7991]
Currently, TextGridObject synchronizes all methods that access metadata on the TextGridObject instance. This may lead to deadlocks when forcing reloadMetadata() operations to a background job as happens with LongOperation, see TG-563 - thus I disabled forcing to background in this specific case.
TODO: Investigate and implement a better synchronization method (with a mutex object that can be passed around) and re-enable forcing to background here to prevent repository access from the UI thread.