implement XEP-xxxx: Message Displayed Synchronization

...in/java/eu/siacs/conversations/entities/Conversation.java

...a/eu/siacs/conversations/generator/AbstractGenerator.java

...in/java/eu/siacs/conversations/generator/IqGenerator.java

...ain/java/eu/siacs/conversations/parser/MessageParser.java

...u/siacs/conversations/services/XmppConnectionService.java

src/main/java/eu/siacs/conversations/xml/Namespace.java

...main/java/eu/siacs/conversations/xmpp/XmppConnection.java

.../java/eu/siacs/conversations/xmpp/pep/PublishOptions.java