Refactor SharingManager so its events provide message header

...pi/src/org/briarproject/api/blogs/BlogSharingMessage.java

...g/briarproject/api/event/BlogInvitationReceivedEvent.java

...roject/api/event/BlogInvitationResponseReceivedEvent.java

.../briarproject/api/event/ForumInvitationReceivedEvent.java

...oject/api/event/ForumInvitationResponseReceivedEvent.java

...c/org/briarproject/api/event/InvitationReceivedEvent.java

...iarproject/api/event/InvitationResponseReceivedEvent.java

...rc/org/briarproject/api/forum/ForumInvitationRequest.java

...c/org/briarproject/api/forum/ForumInvitationResponse.java

...i/src/org/briarproject/api/forum/ForumSharingMessage.java

...i/src/org/briarproject/api/sharing/InvitationRequest.java

...pi/src/org/briarproject/api/sharing/SharingConstants.java

...-api/src/org/briarproject/api/sharing/SharingMessage.java

...src/org/briarproject/sharing/BlogInviteeSessionState.java

.../src/org/briarproject/sharing/BlogSharerSessionState.java

.../src/org/briarproject/sharing/BlogSharingManagerImpl.java

...rc/org/briarproject/sharing/ForumInviteeSessionState.java

...src/org/briarproject/sharing/ForumSharerSessionState.java

...src/org/briarproject/sharing/ForumSharingManagerImpl.java

...-core/src/org/briarproject/sharing/InvitationFactory.java