Notifications new shared folder and new peered device

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Notifications where user intervention is necessary should be implemented for the following cases:

• A new folder is shared
• A new device is peered

Enhancement is based on: https://forum.syncthing.net/t/syncthing-for-macos/2745/22

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Initial prototype is functional to translate events to OS X notifications:

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Had some discussion on the official forum about event behaviour in the webgui:

https://forum.syncthing.net/t/unexpected-continuous-devicereject-events-from-the-api-with-incrementing-ids

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We should keep issue syncthing/syncthing-android#679, and syncthing/syncthing#4084 in mind when implementing this.

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Since v0.14.51 the new API is available so probably we should reimplement my work from branch https://github.com/syncthing/syncthing-macos/tree/feature/issue-52 into Swift.