Add OMEMO policy

There is 3 policy:

- manual: OMEMO session are only started manually
- automatic: OMEMO session are only started if they have been started
  manually before
- always: OMEMO session are always started unless they have been ended
  manually before

Closes #1040 and fixes #1052

src/ui/chatwin.c

@@ -50,6 +50,9 @@
 #ifdef HAVE_LIBOTR
 #include "otr/otr.h"
 #endif
+#ifdef HAVE_OMEMO
+#include "omemo/omemo.h"
+#endif
 
 static void _chatwin_history(ProfChatWin *chatwin, const char *const contact);
 
@@ -73,6 +76,13 @@ chatwin_new(const char *const barejid)
         }
     }
 
+#ifdef HAVE_OMEMO
+    if (omemo_automatic_start(barejid)) {
+        omemo_start_session(barejid);
+        chatwin->is_omemo = TRUE;
+    }
+#endif
+
     return chatwin;
 }