TreeDataProvider is called even if view is not visible

[weinand]

• create an extension that contributes a TreeView
• create a TreeDataProvider that fires _onDidChangeTreeData(undefined) events every 2 seconds
• run the extension
• collapse the TreeView to make it invisible

Observe:

• TreeDataProvider.getChildren is called even if view is insible

I would expect that TreeDataProvider.getChildren is only called if the view is (or becomes) visible.
If this cannot be achieved we need an new event that tells us when the view is not visible so that we can stop firing _onDidChangeTreeData.

Without this it is not possible to have a view that consumes no cpu cycles when not being visible.