notifications: Implement per app grouping

This is the first step of implementing grouping of notifications by app, this doesn't consider threading of conversations at the moment.

This is based on !3103 (merged)

https://teams.pages.gitlab.gnome.org/Design/web-experiments/static/notifications-grouping/ https://gitlab.gnome.org/Teams/Design/os-mockups/-/tree/master/notifications-calendar?ref_type=heads

There are still some things missing, a no exhaustive list:

• Proper key focus navigation
• Gray out area around expanded group (needs design)
• Hover state for collapsed group
• MediaMessage (MPRIS) isn't disabled when a group is expanded
• CSS is a little bit messy
• In some cases there are The actor '' is currently inside an allocation cycle; calling clutter_actor_queue_relayout() is not recommended warnings depends on !3059 (merged)
• Order notifications and groups based on time
• Group notifications by app id or source title, if no app id (currently used source title) We can group simply by Source object.