Use statefs to manage context properties in Qt5 version

Centralize context property handling to class timed. The context properties /com/nokia/time/time_zone/oracle and /com/nokia/time/system_time/operational appear to be obsolete, so they have not been ported to Qt5.

Note that statefs does not support other data types than bool and string, so the Qt5 version publishes the active alarm queue(context property Alarm.Trigger) as a string instead of a QMap<QString,QVariant>.