[FEATURE] BackgroundTask: replace legacy inline progress bar

[thibsy]

Hi @thojou,

This was my rather desperate attempt of replacing the inline legacy progress bar with the new UI component (Progress\Bar). However, I could not manage to replace this yet, because funding dried out and some issues came to light.

The main issue is that the whole process of how these notification items are updated is not compatible with the progress bar anymore. Until now, the entire notification item was updated async every X seconds, which delivered an updated version of the legacy progress bar. This is not possible anymore, because the new progress bar can only be updated on the client via async requests, and since they are part of the notification item which should be replaced entirely, this would break the progress of the progress bar. I managed to work around this for "normal" tasks, but I did not find a solution for the "user interaction" ones. But this was mostly due to the fact that – for the life of me – I could not manage to implement a working example with a user interaction.

The PR also contains a possible approach for registering update listeners to the new progress bar, which would allow to trigger a replacement request for the "user interaction" tasks once they are done, to fully either replace surroundings of the progress bar. But I could not look into this anymore.

So next steps would be to check if the approach using update listeners is feasible to replace the "update the notification every X seconds" concept.

Since this cannot be merged, feel free to just close this PR.

Kind regards,
@thibsy