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.
Edited by Fushan Wen