applets/notification: port NotificationItem to anchor-based positioning

Layout + ScrollView produces unpredictable results, and since ScrollView
depends on the content height to adjust its size properties, but
re-layout will create unnecessary size adjustments for ScrollView in
Layout, there will be many binding loop warnings in the log.
Anchor-based positioning does not have those issues and is more
efficient.

Test: run

while [ 1 -eq 1 ]; do notify-send -a "Notification Test" "Evil Layout" "$(openssl rand -base64 64)";sleep 1;done

and watch the log of plasmashell.

BUG: 445459
FIXED-IN: 5.27