[alarms] Update alarm enabled status when alarms are triggered

Connect to timed signal alarm_triggers_changed, which lists
the currently active alarm trigger times. Update alarm
enabled status based on which alarms are active.

The processing of the information provided by timed is delayed
500 ms as timed may emit alarm_triggers_changed signals in
rapid succession.
[alarms] Add support for countdown alarms
Clarify documentation for countdown alarms
[alarms] Add flag onlyCountdown to filter AlarmsModel contents

If onlyCountdown is true, then the AlarmsModel includes only
countdown alarms. If onlyCountdown is false, then the model
includes only regular alarms. Changing the property will
repopulate the model.
Merge pull request #4 from pgerdt/countdown-alarms

Countdown alarms, update alarms when they are triggered