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 `execute` method on commands now has a second range parameter. The composite command loops through all selected ranges and calls execute on each subcommand for each range. Still need to update the `.` address to work with the new scheme.
Scroll methods are now Editor.scrollTo(), Editor.scrollHorizontally() and Editor.scrollVertically()
Editor.scrollTo() can only be called once per runloop
