fix(linux/xdgportal): Properly support multiple screens by exposing pipewire streams as separate displays

    Enables display switching on the client-side with Sunshine's existing
    shortcuts (CTRL+ALT+Fxx) when selecting multiple screens on the
    screencast source selection dialog (or automatically all availabe screens
    when using a combined remotedesktop+screencast session, tested with KDE).
    Each screen (selected) stream given by the portal will be available as
    a separate display (consistently ordered by position).

Author: Kishi85