Check on napari, if the plugin manager is busy and provide a dialog on close

[goanpeca]

Avoid closing napari if the plugin manager is busy

ref: https://github.com/napari/napari/blob/eab7661459e70479c7c7d587a36463f3b099b64a/napari/_qt/qt_main_window.py#L442

[jaimergp]

Short-term I guess we would need to poll napari directly. Long-term I envision an API in napari where plugins can register their long-running tasks (with a predefined expiry date, so maybe it's a matter of "keeping them alive" as needed) and would also unregister them once done. Then napari would check that list when needed (e.g. upon closing), and could even be exposed somewhere in the UI for more information.

[jaimergp]

The status payload would look something like this:

{
  "status": "some-value-from-enum",
  "description": "additional details for a tooltip or whatever",
  "timestamp": "ISO-something for the moment the payload was sent (or received; delegated to napari?)",
  "id": "some sort of unique identifier to retrieve this later and remove it"
}