Avoid UI access in async viewers which operate on non UI threads #710

[jonahgraham]

(Backport of #712 for the 4.29 branch)

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

[jonahgraham]

It looks like there is some housekeeping needed on the branch before I can get a clean build. If someone is happy to merge this change as is that would be great. Otherwise I may try to find the time.