nbconvert --to script

Use extension and mime type from NB metadata
'Download as' script
Update docs about kernel_info_reply
Fix asyncy nbconvert to download
Refactor the do_diff and manual child view lists into a separate ViewList object
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