Progress Notifications API

### Problem

SysTray apps were phased out but one experience that wasn't taken into account were file syncing services. There isn't a solution for this displaying sync progress data with as much detail as these apps did.

### Proposal

My solution to this would be progress notification APIs. These would build upon [the ability to replace a notification through referencing its ID](https://docs.elementary.io/develop/apis/notifications#replace) and push the idea further with the addition of progress data.

So what ends up happening is that as progress gets updated, the same notification will be swapped with a new one with up to date progress data over time.

Unlike progress indicators in the dock, it will be possible for there to be multiple notifications to have progress data from the same app.

### Prior Art (Optional)

https://docs.microsoft.com/en-us/windows/apps/design/shell/tiles-and-notifications/toast-progress-bar