Xpra: Ticket #2171: support upgrade-desktop

At the moment, to upgrade a desktop server, one has to use xpra exit followed by xpra start-desktop --use-display

Sun, 24 Feb 2019 04:49:31 GMT - Antoine Martin: status changed; resolution set

Done in r21847.

Sat, 23 Jan 2021 05:44:06 GMT - migration script:

this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2171