Renamed "user-defined types" as "structs" in the serialisation format.

api/net/sf/briar/api/protocol/Types.java

api/net/sf/briar/api/serial/Reader.java

api/net/sf/briar/api/serial/SerialComponent.java

api/net/sf/briar/api/serial/Writer.java

components/net/sf/briar/protocol/AckReader.java

components/net/sf/briar/protocol/AuthorFactoryImpl.java

components/net/sf/briar/protocol/AuthorReader.java

components/net/sf/briar/protocol/BatchReader.java

components/net/sf/briar/protocol/GroupFactoryImpl.java

components/net/sf/briar/protocol/GroupReader.java

components/net/sf/briar/protocol/MessageFactoryImpl.java

components/net/sf/briar/protocol/MessageIdReader.java

components/net/sf/briar/protocol/MessageReader.java

components/net/sf/briar/protocol/OfferReader.java

components/net/sf/briar/protocol/ProtocolReaderImpl.java

components/net/sf/briar/protocol/RequestReader.java

...nents/net/sf/briar/protocol/SubscriptionUpdateReader.java

components/net/sf/briar/protocol/TransportUpdateReader.java

components/net/sf/briar/protocol/writers/AckWriterImpl.java

...onents/net/sf/briar/protocol/writers/BatchWriterImpl.java