[enhancement][tree] All tree widgets automatically select either the root or its first child if any

[kittaakos]

Here is the related code: https://github.com/theia-ide/theia/blob/67bc2d4e9ebb0dd169005f80fdeb41074f61463a/packages/core/src/browser/tree/tree-widget.tsx#L247-L257

Subclasses cannot customize it conveniently. (At least I am not aware of it, if you know, let me know)

Extract the logic to a protected method, so that subclasses can adjust the behavior.

🎖 Bonus: introduce an optional property on the TreeProps for it.

(Why is it called as a TreeProps and not TreeWidgetProps? Confusing.)