backport: alarms: Make alarms persisted and add missed alarms notation

This stores the alarm state as well as the alarm and snooze time
to gsettings. This alllows us to tell the user about missed alarms.
A user could miss alarms for two reasons:
- Clocks wasn't running when the alarm went of.
- The user didn't stop the alarm within "ring-time"

This also adds the baseline for starting Clocks via systemd user .timer
units by making Clocks checks at startup if any alarms need to go off.

As a side effect: closing Clocks doesn't stop an alarm and when Clocks
is reopend within "ring-time" after an alarm or snooze the alarm starts
ringing again.