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
• Persistent notification while the timer is active #236