Refactor the do_diff and manual child view lists into a separate ViewList object
allow unrecognized outputs and cell types in notebooks from the future

in nbformat schema / validation
Handle unrecognized outputs and cells from the future
Make error messages more consistent
Make state_change initialization more conventional
Missing semicolon
Make a copy of the input list so that changes to the children list don't automagically change our reference list
tweak documentation for viewlists
Basic infrastructure for new texteditor component
Loading a file works
Saving files works
Refactor editor into Editor class
Enable codemirror search/replace shortcuts
Update text editor for new contents API
Override Ctrl-S shortcut to save text editor contents
Use NotificationArea in the text editor
Set page title for editor
Styling to make it clearer where the editor is
Better way of saving through CodeMirror

This should support whatever keyboard shortcuts are configured for the
'save' command.
Implement view logic in selection containers.