Added lastUpdated/lastChanged tracking to core item framework

Kai/Thomas - this is a change to some core code so will need close inspection! It is a simple addition to the Item framework which should allow much easier tracking of when an item was last updated/changed. This can be partly achieved already using the persistence framework, but for things like temp sensors which are updated very frequently persistence would mean a large data store, whereas this simple change allows you to check if a sensor has been active recently. Also - not even sure if the persistence stuff currently supports persisting 'updates only'? Feel free to reject if you feel this isn't in line with your design, but it would be useful to a few of us to include this simple change.