WIP multi-user feature

When a file is edited, an UUID is given and memorized at server's side. So, an other user/page/navigator opened on the same file, shouldn't be able to be edited, but, at this step of the WIP, It return -1 as UUID. Have to discuss about if we allow server's call at each keyboard entry to bypass the issue of closing page/navigator which we cannot retrieve the exit confirmation answer. If yes, so may we extend the initial feature wished, like allow multi-user edition on the same instance.