playback/player: Call gst_deinit() in all applications at the end of main()

Needed to be able to track leaks using valgrind or the leaks tracer.

playback/player/gst-play/gst-play.c

@@ -768,5 +768,6 @@ main (int argc, char **argv)
   play_free (play);
 
   g_print ("\n");
+  gst_deinit ();
   return 0;
 }

playback/player/gtk/gtk-play.c

@@ -1910,5 +1910,6 @@ main (gint argc, gchar ** argv)
   status = g_application_run (G_APPLICATION (app), argc, argv);;
   g_object_unref (app);
 
+  gst_deinit ();
   return status;
 }

playback/player/qt/main.cpp

@@ -30,6 +30,7 @@
 int main(int argc, char *argv[])
 {
     QGuiApplication app(argc, argv);
+    int result;
 
     QCommandLineParser parser;
     parser.setApplicationDescription("GstPlayer");
@@ -70,5 +71,8 @@ int main(int argc, char *argv[])
     if (!media_files.isEmpty())
         player->setPlaylist(media_files);
 
-    return app.exec();
+    result = app.exec();
+
+    gst_deinit ();
+    return result;
 }