Allow opening tree view collapsed

The tree view is a nice feature of datatable, I really like it. However, I'd like to open the tree collapsed by default. (I have a large tree, so it's nicer to expand only the parts that you want to look into.) So it would be nice to have a config option that states whether the tree should initially be expanded or collapsed. (You could even make it more advanced by stating up to what level the tree should be expanded, but for my use case just a boolean would be fine.)

Start by locating the tree view expansion logic and the existing configuration options for datatable. Add a boolean setting for the initial expanded state, then verify that the default remains expanded and that collapsed initialization expands correctly when requested.

Written by the indexing model from the issue text.