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/core.c

@@ -763,7 +763,7 @@ ui_room_join(const char *const roomjid, gboolean focus)
 {
     ProfMucWin *mucwin = wins_get_muc(roomjid);
     if (mucwin == NULL) {
-        mucwin = (ProfMucWin*)wins_new_muc(roomjid);
+        mucwin = mucwin_new(roomjid);
     }
     ProfWin *window = (ProfWin*)mucwin;