Reveal cursor on panel expand

@peterflynn Simple fix for issue #3003, and (nearly) identical to closed and outdated pull request #3931.
In general this seems to be baking into Editor a lot of knowledge about the panel layout surrounding the editors.  This knowledge should really be centralized in PanelManager... especially since we'll soon need to interject the new EditorLayoutManager module in between them (e.g. in a 2x2 grid view, the assumptions here about how panel height affects editor height will no longer be true).

Could we do something like pass in an editor-height delta to this function, which for now is computed by PanelManager?  (Then we could name it something like `showCursorAfterResize()`).  Or perhaps even have `setSize()` call this for us (that way it can accurately compute a delta, and if we need any pre-resize measurements we could actually take them before the resize has been done.  (We'd presumably want an optional new arg to `setSize()` to trigger this, since things like window resizes shouldn't do this).