wayland: Add support for the persistence feature of the remote desktop portal

Same idea as kdeconnect-kde!639 (merged), v2 of the remote desktop portal can accept and return a restore token that we can use to avoid constantly asking the user for permission every time Krfb is started.

Note that there's a bug in xdg-desktop-portal-kde that breaks persistence if it's restarted for whatever reason, for example when rebooting (see https://bugs.kde.org/show_bug.cgi?id=480235 and kdeconnect-kde!639 (comment 859651) for more details), so at the moment this only works when restarting Krfb in the same session.