New plugin (token) for aria-live

[krassowski]

Problem

Better screen-reader integration was requested/proposed in a few issues/pull requests:

• Usability with screen readers #4878
• Screen reader with keyboard shortcuts #6573
• Added aria-live to commands to improve screen reader accessibility.  #15048
• Added aria-live announcement when a notebook cell is taking a long time to process.  #15554

However, there is repetitive code in #15048 and #15554 which writes directly to DOM elements to cause the side-effect of reading out text by screen reader. As highlighted in the reviews:

• this needs customization such as the delay for #15554
• the entries need to be cleaned at some timeout to avoid memory leak
• user needs to be able to toggle this feature off

Proposed Solution

Create a new plugin with a public API to announce() a message to screen users, and with settings to provide customisability.

Additional context

None

[krassowski]

I'm not quite sure where this plugin should live. So far the aria-live region element was proposed to be added to the shell in application package, which is good in the sense that everyone gets access to it, but is not great for reusability (we would want to have it in Notebook to, but notebook has a different application).

A new package is an option, though it seems a bit slim, so maybe apputils?

[krassowski]

Note: cross-check with interface for jupyterlab logger and with the announcements API