Handle 'deletable' cell metadata
Add tests for undeletable cells
Make cell be undeletable ONLY when metadata is explicitly false
Fix tests
Handle NoSuchKernel errors more gracefully
Show the user a different notification
Report the exact error that occurred
Remove 'we' from message
Better user experience when kernel isn't found
Use 501 error code instead of 400
Return a proper JSON object
Always show the modal dialog, and have a fallback generic message
Don't pop up modal dialog
More descriptive short message
s/long thing/set_state
s/_handle_receive_state/set_state in tests too
s/apply_update/set_state
support `%gui qt5` in kernels

same as terminal IPython in #6496
didn't need anything beyond register
support `%matplotlib qt5`

just needed the key, since `%gui qt5` already works
support %matplotlib nbagg

we don't need to do anything, just add keys to a couple of dictionaries