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

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 3b9c02c 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 eclipse-platform#710

debug/org.eclipse.debug.ui/ui/org/eclipse/debug/internal/ui/viewers/AsynchronousViewer.java

@@ -414,6 +414,11 @@ protected Widget doFindInputItem(Object element) {
 	protected void doUpdateItem(Widget item, Object element, boolean fullMap) {
 	}
 
+	@Override
+	public void refresh() {
+		refresh(getRoot());
+	}
+
 	@Override
 	protected void internalRefresh(Object element) {
 		// get the nodes in the model