API to programatically expand/collapse tree view

### Request An API on TreeView which allows extensions to expand or collapse their contributed views within the view container. That is, expand/collapse the view itself, not the items in the view. ### Example Use Case If I have a view container with multiple contributed views, not all views may be relevant all the time. It would be nice to be able to close the irrelevant views, to give more screen real estate to the relevant ones. For example, maybe one view shows the definition of the currently selected word in the editor: if nothing in the editor is selected, then ideally this view could just be collapsed since it is providing no value to the user. When something is selected, it should be possible for the extension to re-expand the view.