TreeGrid expand api should check internally for multi select column

[stefanuebe]

The TreeGridElement provides shortcut api to handle expansion (check if expanded, expand, collapse, etc), e.g. expand(int). Unfortunately these methods do not take multi selection into account. Having multi select active leads to a no such element exception, since the grid checks the first column for the tree grid toggle.

I know, that from a technical perspective, in this case the first column is indeed not the hierarchical column, but since the multi select column is not a "normal" dev defined column, I think that we should handle that use case for the devs.