Add offline manager

[rolson]

• [x] user notification should be fired at the app level, example app can do this so that app devs can just copy/paste the code
• [x] it's best if the developer does not need to interact with the job manager to use the component
• [x] it would be nice to have a WindowGroup/Scene modifier that is a one stop shop for the user to wire things up for the offline toolkit component(s)

Note: the user will still have to add the required job manager plist settings to the app, there is no way to workaround that requirement.

The approach taken in this PR fixes a few lifetime issues. One main one, consider this case:

• app starts up, user opens OfflineAreasView, starts downloading a job.
• app is backgrounded and killed at some point, doesn't matter how
• app restarts, either by user or by the OS, doesn't matter how
• any code dealing with a job completion will not be run unless the view OfflineAreasView is re-shown.
• this might include removing the job from the job manager or showing a notification that the job was complete

This PR fixes the above situation by introducing an always-alive OfflineManager singleton, which is initialized through the offlineManager(...) Scene extension method. Now when the app is restarted and a job completes, you don't need to open the OfflineAreasView to run the completion code.

[rolson]

Looks good. OfflineMapAreasExampleApp.AppDelegate could be removed if we decide that only background and not foreground notifications are needed.

I have another PR that I will open after this with some logging added and a bug fix, and I will remove the file in that PR.