Prettify notifications + Add Notification Center

[Aproxia-dev]

Make the notifications look more acceptable + Add a notification center to keep track of missed notifications

• Notifications should be tracked even in DND mode
• Notifications shouldn't appear in DND mode
• The notification center should be disabled in DND mode
• Applications should have disabled/disableable notifications (Separately as notifications and in the notification center)
• Maybe add something like GitHub notifications or add Buttons (from Add a dashboard #4) to the Notification Center to make it look less empty