3rd part of the change started by D28882 and D28884.
Instead of implementing KWayland::Server::RemoteAccess, we implement
KWayland::Server::Screencasting and deal with it with ScreenCastStream
that used to be in xdg-desktop-portal.
This allows us to offer video streams directly from kwin rather than
building them from the outside (be it xdg-desktop-portal or otherwise).