gtk4: Add python example

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1259>

video/gtk4/examples/gtksink.py

@@ -0,0 +1,57 @@
+import gi
+import sys
+
+gi.require_version("Gtk", "4.0")
+gi.require_version('Gst', '1.0')
+
+from gi.repository import Gtk
+from gi.repository import Gst
+
+Gst.init(sys.argv[1:])
+
+gtksink = Gst.ElementFactory.make("gtk4paintablesink", "sink")
+# Get the paintable from the sink
+paintable = gtksink.props.paintable
+
+# Use GL if available
+if paintable.props.gl_context:
+    print("Using GL")
+    source = Gst.ElementFactory.make("gltestsrc", "source")
+    glsink = Gst.ElementFactory.make("glsinkbin", "sink")
+    glsink.props.sink = gtksink
+    sink = glsink
+else:
+    source = Gst.ElementFactory.make("videotestsrc", "source")
+    sink = gtksink
+
+pipeline = Gst.Pipeline.new()
+
+if not pipeline or not source or not sink:
+    print("Not all elements could be created.")
+    exit(-1)
+
+pipeline.add(source)
+pipeline.add(sink)
+source.link(sink)
+
+def on_activate(app):
+    win = Gtk.ApplicationWindow(application=app)
+    box = Gtk.Box(orientation=Gtk.Orientation.VERTICAL)
+
+    picture = Gtk.Picture.new()
+    picture.set_size_request(640, 480)
+    # Set the paintable on the picture
+    picture.set_paintable(paintable)
+    box.append(picture)
+
+    btn = Gtk.Button(label="▶/⏸")
+    box.append(btn)
+    btn.connect('clicked', lambda _: pipeline.set_state(Gst.State.PAUSED) if pipeline.get_state(1)[1]==Gst.State.PLAYING else pipeline.set_state(Gst.State.PLAYING))
+
+    win.set_child(box)
+    win.present()
+
+app = Gtk.Application()
+app.connect('activate', on_activate)
+
+app.run(None)