Add stop button to example.

Author: ryanheise

just_audio/example/lib/main.dart

@@ -185,6 +185,11 @@ class ControlButtons extends StatelessWidget {
             }
           },
         ),
+        IconButton(
+          icon: const Icon(Icons.stop),
+          iconSize: 64.0,
+          onPressed: player.stop,
+        ),
         // Opens speed slider dialog
         StreamBuilder<double>(
           stream: player.speedStream,