add connection_get_jid()

Use a singleton `Jid` inside the connection instead of always re-creating
a `Jid` from the same string.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

src/xmpp/xmpp.h

@@ -193,7 +193,8 @@ jabber_conn_status_t connection_get_status(void);
 char* connection_get_presence_msg(void);
 void connection_set_presence_msg(const char* const message);
 const char* connection_get_fulljid(void);
-char* connection_get_barejid(void);
+const Jid* connection_get_jid(void);
+const char* connection_get_barejid(void);
 char* connection_get_user(void);
 char* connection_create_uuid(void);
 void connection_free_uuid(char* uuid);