Colour the notification indicator based on notification status

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An always-red notification indicator won't help much at first sight, and it can make users confused.
Then it can be better to change the indicator to different colours based on the overall status like this:

• When we have in-progress tasks, can tint the indicactor blue, and show a spinning circle instead of plain numbers.
• For notifications with low priority (like screenshots, download notifications), can use another colour a little darker than red.
• For high priority notifications (like exceptions or update reminders), we can keep using the current red colour.