envy24control: simplify code by using gtk_widget_show_all

alsa-project/alsa-tools pull request #29 was opened from andreas56:

Remove all gtk_widget_show calls and replace it with just one gtk_widget_show_all. This makes the code shorter and it will also help a future port to GTK 4, where gtk_widget_show is gone.

Request URL : https://github.com/alsa-project/alsa-tools/pull/29 Patch URL : https://github.com/alsa-project/alsa-tools/pull/29.patch Repository URL: https://github.com/alsa-project/alsa-tools