Countdown alarms, update alarms when they are triggered

Add support for countdown alarms, update alarm status by listening to the alarm_triggers_changed signal emitted by timed. 

Timed signal listening and countdown alarms are added in separate commits, it may be helpful to examine them separatedly, starting commit	ca78ae6 / "Update alarm enabled status when alarms are triggered".

Timed changes have been merged with this pull request: https://github.com/nemomobile/timed/pull/28.