Xpra: Ticket #1799: request access to session from current clients

Originally recorded in #1690 which is now dealing with shadow mode only via an authentication module. Follow up from #1368.

Give users greater control over when sessions are shared / stolen / locked: the server can forward the request to the existing client(s) if they support the new feature, delay the new client's connection timeout, and depending on the response from the existing client(s) either allow the connection or reject it.

We could then start the sessions with sharing disabled and let the users decide at runtime when to allow others to connect. The dialog could have a "lock" option to prevent future dialogs from popping up, by setting the lock flag to true.