[timed] API for getting a signal when alarm queue changes

Add public api for subscribing to a signal providing currently
active alarms which is emitted every time the alarm queue
changes.

See documentation for alarm_triggers_changed_connect(), and
alarm_triggers_changed_disconnect() in src/doc/libtimed.3.txt
for more information.
Merge pull request #28 from pgerdt/alarm-queue-change-signal

[timed] API for getting a signal when alarm queue changes