Cockpit should explicitly announce that an extension-offered widget is available

[rafaellehmkuhl]

Today, Cockpit already fetches widgets from the extensions, but it only shows them in the edit-mode list.

We should announce them in a dialog when Cockpit boots, similar to what we do with the extension-offered Actions, and allow the user to add it to its view with a single click.

Once they interact with that dialog, we don't need to do it anymore.

The use-case is specially for the Radcam extension, regarding its Radcam Configuring Widget.

[rafaellehmkuhl]

This should be much easier to add after #1962.