Reveal cursor on panel expand

@peterflynn Simple fix for issue #3003, and (nearly) identical to closed and outdated pull request #3931.
@peterflynn actually, layout is computed in the Editor in couple of other places as well (see `centerOnCursor`). Also, note that the actual panel size is not involved. I think it's only natural for PanelManager to signal the layout changes and for the editor to decide where to scroll, which is what we were trying to achieve.

You're right split view will affect this. I'll take a look at current work to see how the bottom panels are being managed.