Expose wl_display_set_global_filter as a virtual method

Summary:
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.

Test Plan: Attached unit test

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: graesslin, plasma-devel, #frameworks

Tags: #frameworks, #plasma

Differential Revision: https://phabricator.kde.org/D8050