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



We want to do this last after the document has been opened and the pane has changed its current file. This prevents unwanted flicker in the title bar.  If we do this before the document has been opened and the current file changed in the pane then the activating the pane will cause the title bar to change again after opening the document