Working Set Performance Improvements

This is for #9439 -- Clicking on items in the working set is slow.

* Don't create the drag ghost until needed
* Improve `Pane.focus `(one big drag was `currentElement.blur()` so we only need to do that if we're focusing something that isn't code mirror.
* Improve `EditorManager._showEditor()`, the UX changes for the pane DOM weren't targeting the right node to see if it needed to re-add the editor to the pane so this was happening every time the editor was shown.
* Improve `_open` and `_edit` to not activate pane until *after* opening the document

Also fixes #9428



Also, it's not clear how this prevents pane redundancy checking, so can you add a comment that gives a brief explanation?