in-app-notification: move notifications from top to bottom of the window

As described in #1022 (closed) , having in-app-notifications pop up at the top of the screen means sometimes blocking email header info. This moves it to the bottom of the window (still in the center, though), so that it only blocks part of the message

You can see that here:

That was the easiest change to make. If we want to get fancy, or if this ends up being a problem with certain screens, the GtkOverlay docs say that we do more complicated placement with the get-child-position signal and margin properties. Hopefully keeping it simple by just placing it at the bottom works, though!