The class remained package-private, even after the experimental marker
was removed with 8fb6198fecaf93552ac7007e9104360adfdca565, seemingly due
to an oversight.
This is reinforced by the fact that the TreeColumnViewerLabelProvider,
which directly extends the TableColumnViewerLabelProvider is public.
This leads to weird situations where one has to use the
TreeColumnViewerLabelProvider for both TreeViewers and TableViewers,
even though its name would only suggest one is compatible...
Resolves #1335