FillDirectMessageRecipients: after the plugin populates the recipients select-box; federation plugins must add their own recipients; note that only subscriptions should be added
- User $current: Currently logged user
- array &$recipeints: Profiles to be shown in the select-box

DirectMessageProfilePageActions: when about to show the direct message button in someone's profile; federation plugins must validate their users otherwise the button is ommited
- Profile $target: Profile receiving the message button

SendDirectMessage: after storing a new private message; federation plugins must distribute the message to the remote profiles
- Notice $message: Message to be distributed