Add option to configure Always on Top behavior for notifications

[justin-wtower]

I've been testing and just went through an upgrade workflow - the notification that pops up while the update is preparing (25-30 minutes) is always on top, so while the user should be able to keep working, it's kind of disruptive to have this dialog blocking a portion of the screen. I looked at the code (v5.0.0b5 line 9315-9317) and it looks like the -always-on-top option is always passed to Notifier. It would be nice if we had a config option to specify which notifications should be forced to be always on top otherwise the notification would be able to be forced behind the active application so it would be less disruptive to the user's work. This config option would be similar to the functionality of the DisplayNotificationsCentered preference where listed notification types would be always on top and the rest of the notifications would not be forced on top.