adding update notification checking - this requires autopsy-updates.xml ...

Adding update notification checking - this requires autopsy-updates.xml to be uploaded to sleuthkit.org.  To actually trigger an update notification, the Netbeans platform is only looking for this file to be updated...hence no need for specific modules to be referenced.  When a new release is available, we should just update the timestamp and/or message on this file and a notification popup in the lower-right will show up for anyone running Autopsy with the notification center enabled (will be by default).  Once the notification is close by the user, it will no longer appear until the next time the file is updated.