frontends (tools/webrtc): implement screensharing for Wayland + bug fixes: - Freedesktop Desktop Screenshare port is now used when Wayland is detected (needs `xdg-desktop-portal` with the implementation corresponding to desktop environment). - Add a binding feature to feedback state to application (e.g. if desktop sharing is cancelled from desktop environment, or at portal's permission request level). - fix misnaming of video source (was wrongly named `camera` instead of `video`). - fix desktop sharing pad selection in `input-selector` when it has been added once, then removed, then added again. rel 434