Use waitFor instead of sleeping.
Remove sleep from the following,

Button, float, int, multicontainer, selection widgets.
Use wait_for_widget in widgets and string widget tests
Update whatsnew doc from PR files
Container classes should default with 'vbox' css
Merge pull request #5010 from jdfreder/widget-ff

Fixes for widget alignment in FF
Merge pull request #5007 from takluyver/pr-whatsnew-3

Update whatsnew doc from PR files
Fix command mode & popup view bug

Added elements property to PopupView
don't write cell.trusted to disk

Notary.check_cells should pop the keys, not just read them.
Add notebook examples link to help menu.
Merge pull request #5015 from minrk/check-cells-pop-trusted

don't write cell.trusted to disk
Merge pull request #5017 from takluyver/help-menus-nbexamples

Add notebook examples link to help menu.
more subtle kernel indicator

add casper.wait_for_busy
like wait_for_idle, but busy

restore kernel notification widget
it's still used for interrupt / restart

use wait_for_busy in check_interrupt test
Merge pull request #4842 from Carreau/fix-JG-complaint

more subtle kernel indicator
Unconditionally register $el with keyboard manager

also s/elements/additional_elements
Merge pull request #5014 from jdfreder/widget-popup-keys

Fix command mode & popup view bug
Merge pull request #4952 from jdfreder/widget-tests

Widget test inconsistencies

add wait_for_widget