KFileItemModelRolesUpdater: Avoid multiple updates for Nepomuk data

KFileItemModelRolesUpdater: Avoid multiple updates for Nepomuk data

The Nepomuk ResourceWatcher emits 3 signals - propertyChanged,
propertyAdded and propertyRemoved. We should only listen to either the
propertyChanged signal or the propertyAdded + Removed signals. There is
no point in listening to all 3 signals. That will just result in
unnecessary updates.

Additionally, we do not need to listen to the resourceCreated signal.
That is only emitted when we are watching for a specific types, which we
are not.

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