Every command returns an array of buffer ranges to select from its execute method. The composite command loops through each current selection with each command, then sets the new selection to the aggregated buffer ranges returned by all commands at that step.
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
