[Feature request] Add stretch notificiations

[SanderBouwhuis]

I would like to be able to see a notification every n minutes (e.g., 20) to warn me to stand up and look into the distance for 30 seconds (good for my eyes) and do some movements for 30 seconds (good for my circulatory system).

Considering I already use your clock permanently on my desktop, it would be handy to have it either show a notification, or to make the clock flash or so.
Having an optional single 'beep' in case you visually miss it would be nice too.

I guess I can program it myself, but it would be nice to have it integrated into DigitalClock.