Further notification refactor

Expands on !424 (merged) splitting the PhoshNotificationBanner (formerly known as PhoshNotification)

PhoshNotificationBanner

Now just a simple PhoshLayerSurface that slides into view

PhoshNotificationFrame

A box for showing notifications, contains the app icon/name header

Internally it's a GtkListBox and works off a GListModel allowing it to show a group of notifications ready for drawer/lockscreen

PhoshNotificationContent

The GtkListBoxRow that actually shows the notification title/body/image/actions

Also

• Notification ids are now stored in the notification instead of with get/set_data
• ... but the id is largly hidden as we just act on PhoshNotifications now
• PhoshNotifyManager only cares about PhoshNotification (except for triggering a banner)
• Fixes a few style bugs with action buttons

TODO:

• Tests