Improve notification design

[Hanziness]

Notifications sent by the app can have more options, like badge, icon or actions. A few guidelines:

• [ ] Notifications should show the app's monochrome icon as their badge so the user knows which app the notification is from
• [ ] Notifications should periodically update after the timer has expired to remind the user about the expired timer (this should be configurable: eg. have a toggle to enable/disable loud renotification and to set the interval (eg. to update/resend the notification every x minutes))
• [ ] Bring the app to the foreground if the notification is clicked (especially on mobile)
• [ ] Add actions to the notifications to stop the timer or to start the next section
• [ ] Rework notification texts to make them clearer
• [ ] (Optional) explore if vibration patterns help as this can be set, too
• [ ] Remove icon image from the notification, if possible
• [ ] #236