Refactor all period update checking e.g. fetch active timecard, fetch all timecards, to use a new "update service"

[y2kbugger]

Update service should handle

• periodic running of updaters
• visiblity checking
• on vis changed
• holding onto Timer references
• Need to sync disposing of Timer with lifetime of calling component
  • https://stackoverflow.com/questions/3457521/how-to-check-if-object-has-been-disposed-in-c-sharp

API
UpdatePeriodically(bool OnBecomingVisible, Timespan MaxStaleness Hidden, Timespan MaxStaleness Visible)