feat: refactor snackbar out into a oscd-ui component & background plugin

[stee-re]

In an effort to keep the oscd-shell malable, refactor the snackbar from this project out into its own UI component in oscd-ui, that way plugins can use/extend it as required.
Instead of integrating this directly into the shell and defining a documented "notification" event, perhaps it would be best to add this as a background plugin which distro users can opt to us, or perhaps replace with their "better" implementation. User feedback at its simplest can be temporal, automatically vanishing after a preset period, but nicer is providing a mechanisim to persist the most recent notifications, allowing users to look back on what they missed. We would implement the simplest snackbar implementation but by leaving this as a background plugin, would allow others develop something a little richer, like a notifications panel (as previously described).

• [] Refactor snackbar into oscd-ui
• [] Create new background plugin for catching notification events and triggering the snackbar.
• [] Discuss with community if they see the advantage of having a common well defined notification event for this