Add private message visitor.

...rg/briarproject/briar/api/blog/BlogInvitationRequest.java

...g/briarproject/briar/api/blog/BlogInvitationResponse.java

.../briarproject/briar/api/forum/ForumInvitationRequest.java

...briarproject/briar/api/forum/ForumInvitationResponse.java

...arproject/briar/api/introduction/IntroductionRequest.java

...rproject/briar/api/introduction/IntroductionResponse.java

...riarproject/briar/api/messaging/PrivateMessageHeader.java

...iarproject/briar/api/messaging/PrivateMessageVisitor.java

...r/api/privategroup/invitation/GroupInvitationRequest.java

.../api/privategroup/invitation/GroupInvitationResponse.java