ignore spurious error messages when calling ddcutil

[1player]

Fixes #9

I'm new with GJS, I had a hard time telling spawn_async_with_pipes to ignore stderr, but I seem to understand Gio.Subprocess.new is preferrable nowadays, and the code is simpler anyway.

This commit makes so only the stdout is sent to the callback, and stderr, which might include benign error messages, is ignored and doesn't trip the parser.

[daitj]

@1player No worries, I am in the same boat. I have had hard time to find right implemenation because of constant updates to GJS.