status/volume: Hide sliders initially

We update the visibility on state or stream changes, but those
changes may never happen if pipewire-pulse/pulseaudio isn't
available (for example when running as root).

Hiding the sliders is preferable in that case to showing non-working
controls.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2295>

js/ui/status/volume.js

@@ -34,6 +34,7 @@ var StreamSlider = class {
         this._control = control;
 
         this.item = new PopupMenu.PopupBaseMenuItem({ activate: false });
+        this.item.hide();
 
         this._inDrag = false;
         this._notifyVolumeChangeId = 0;