notifications: Handle application specific notification settings

org.gnome.desktop.notifications.application.<name>

• show-banners (see !675 (merged))
• enable (this one can just go on top of !675 (merged) and make sense to have). See !863 (closed)

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• show-in-lockscreen needs support first (for persistent lockscreen notifications we could have banners easily)
• force-expand (we always expand atm)
• enable-sound-alert (we don't handle sound alert at all atm)
• details-in-lockscreen (we don't show notifications on the lock screen atm)