allow --help and config_file classes to differ

override help_classes or config_classes
to allow them to differ from App.classes
only include kernel config in `--help` output

don't write it to a config file that won't be read
ipython_kernel gets its own config file
Allow timeout and click callback
Add documentation to NotificationWidget methods
Add documentation to notification area
Small changes to notification widget
Add tests for notification area and widgets
Make Widget.views be an object indexed by view id
Clarify stickiness of warning/danger notifications
Added missing quote in iframe height property.
Merge pull request #6564 from rossant/iframe-height-quote-missing

Added missing quotation mark in iframe height property
Fixed off by one error in get_prev_cell

Not sure why this was a TODO. Maybe `find_cell_index()` returned zero at one time, but in the browsers I tested, it always returns null if not found.
Fixes #6561 instructions for installing dependencies on zsh/tcsh
Don't catch repr-errors in pretty

allow exceptions to be raised and displayed by the caller
show tracebacks for errors in formatters

instead of FormatterWarnings
handle missing 'text/plain' in displayhook

avoids KeyError when repr fails
Merge pull request #6569 from gcporras/fix_instructions_for_installing_project_dependencies

Fixes #6561 instructions for installing dependencies on zsh/tcsh
Merge pull request #6394 from minrk/repr-fail-traceback

show traceback for broken reprs
Update documentation for functions that can return null