Send a notification on behalf of applications using the unity API to communicate progress

[danirabbit]

In our recent UI study we found that while a significant number of people expect to use the dock to see long-running process information, a significant number of people said they expect to see this information in notifications. This was especially true of respondents not running elementary OS.

It would be worth exploring listening to the launcher API for progress and sending a notification on behalf of applications to display this progress. We probably want to make sure applications can send a description of the progress as well. This should also be supported in the notifications indicator and once progress is completed, we should probably automatically withdraw these notifications so we don't overlap with applications sending notifications about task completion.

Prior art to explore here is KDE Plasma and Android