gst: deviceprodiver: Sort devices by session priority

Applications using the device provider typically list devices in the
order they were added. In order to ensure that apps pick nodes like cameras
with the highest priority by default, sort devices accordingly.

This unfortunately does not not have an effect on nodes added later,
e.g. on hotplug.

Closes https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/3072

src/gst/gstpipewiredeviceprovider.h

@@ -39,6 +39,7 @@ struct _GstPipeWireDevice {
   uint64_t            serial;
   int                 fd;
   const gchar        *element;
+  int                 priority;
 };
 
 struct _GstPipeWireDeviceClass {