Warns and prevents deletions of items that are used

Previous versions of Overseer would sometimes let the end user accidentally delete something important… Most notably, Overseer would allow the user to delete a resource group that has resources in it. This would make those resources inaccessible, even though they’d still be monitored and alerts sent. This is obviously not a good thing, so in Overseer 5.0, I went through all the ‘things’ in Overseer, and added a quick check when deleting them– now if you try to delete a notification, notification group, schedule, resource group, etc. that is in use by another item(i.e. a resource using a schedule for network monitoring), Overseer will prevent you from doing so and explicitly tell you why.