[Feature] add more flexible snooze option for fullscreen alarm

[patrickunterwegs]

A "+5 minute" option would be useful for those times where I need to finish up what I am doing but the reminder soon. 1 hour and 1 day seem too long for that use case. I think my preference would be 5 minutes and 1 hour, and make 1 day require opening the app and manually editing the alarm. Hm, yes, I get it... I'm thinking of making a very flexible option where you can choose minutes or days for a delay. But also here I have second thoughts as it might be better to keep it consistent with the notification...

See #849