Required?: YES (Within a content-handler block)
Max Length: 100 characters
Legal Characters: us-ascii only

The toolbar icon is displayed TBD.XXX

When naming this icon, create two versions, one with _on legal characters: us-ascii only
in the name and one with _non in the name. Configure the _on version in the XML. If the item is then unavailable it is displayed using the _non icon.

If the locale:key is provided as well as a basic path inside the element then the locale:key value is used and the element text is ignored. This key is located in the current locale's bundle as per the bundle action so the images can be localized.