app-window: Move DisplayPage under below_bin stack

This simplifies the widget hierarchy quite a bit and fixes the issue of
notifications going invisible on switch to display page.
app-window: A minor correction/update in a comment

Correct/update name & type of widget in widget hierarchy doc comment.
app-window: Make use of duration settting

After switching away from clutter usage, we were not much using duration
settings which is meant for all transition animations. Thix patches
fixes that regression.