This Replaces #9493

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