This allows a server to filter which globals are visible and bindable by
clients.
Design rationale:
Could be it's own class with Display as an arg, but we need the lifespan
to exactly match Display, and the cardinality to match Display and it
needs to be set after we're started but before clients connect.
Better to enfore rules with code than with documentation.
I'm filtering by interface name as there isn't any other good
identifier of what a wl_global refers to, even if you could assume
you can cast the userdata to a Server::Global.
Note also that I posted my proposal on T4437 of how the kwin side should
behave.
We probably need to restart a discussion / fight to the death on plasma-
devel about that before proceeding.