SelectionViews now update their appearance immediately when constructed. We can't assume they're empty. CursorView doesn't do a blanket `off()` call to its model when it's removed anymore, which was screwing up selection updates when switching back. Only attach selections / cursors when the editor is attached, and extract everything we do into a `renderWhenAttached` method.
The old cursor view still does most of the work, but to add a new cursor view we now must add a cursor model to the edit session at the correct position. The editor watches the 'add-cursor' event on edit session and add a corresponding CursorView to the CompositeCursor whenever a model is added. Next step: have the cursor view observe the cursor model and shadow its changes in position.
