gtk4paintablesink: Also create window for gst-play

So it can be easily tested with
```
gst-play-1.0 --videosink=gtk4paintablesink ...
```

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1555>

video/gtk4/src/sink/imp.rs

@@ -297,6 +297,7 @@ impl ElementImpl for PaintableSink {
         match transition {
             gst::StateChange::NullToReady => {
                 let create_window = glib::program_name().as_deref() == Some("gst-launch-1.0")
+                    || glib::program_name().as_deref() == Some("gst-play-1.0")
                     || std::env::var("GST_GTK4_WINDOW").as_deref() == Ok("1");
 
                 if create_window {