Add a way to delete user activities/spotlight indexed items

[marcpalmer]

By specifying an app-unique persistentIdentifier on activities, the NSUserActivity instances can be deleted from the system's database. For example if you delete a document, activities related to it should be removed to avoid frustrating results.

Currently persistentIdentifier must be set manually, but we could:

• Automatically generate an ID based on the action type name (though this might change over time... so should default to this but allow supplying a specific value by convention not just for use with this feature - uniqueIdentifier?) plus some hash or representation of the input
• Add a deleteActivity(forInput:x) call on Action that will recreate this same ID and call deleteSavedUserActivitiesWithPersistentIdentifiers

The latter, plus the nuclear deleteAllSavedUserActivitiesWithCompletionHandler are the only options available, so I don't think there's more we can do in convention terms related to actions.

See https://developer.apple.com/documentation/sirikit/deleting_donated_shortcuts