The AsynchronousViewer class hierarchy reuses the Viewer interface
but adds additional constraints on it, in particular it allows many
of its methods to be called from non-UI threads.

See javadoc summary:

https://github.com/eclipse-platform/eclipse.platform/blob/e92b72761b8543358ba9811ea49807f75e98bad5/debug/org.eclipse.debug.ui/ui/org/eclipse/debug/internal/ui/viewers/AsynchronousViewer.java#L57-L72

Commit 3b9c02c4e1f8364bfb05f66fc5d046ab4d2ff8a2 added UI access to method
that used to not have any, therefore this patch restores that non-UI
implementation locally consistent with other methods in the async viewer
that operate on the model.

Fixes https://github.com/eclipse-platform/eclipse.platform/issues/710