system-monitor-graph@rcassani: Fix Stuttering

[schorschii]

Hi @rcassani , we noticed that subprocess.init(null) causes microstutters on our desktops, see #932. This PR fixes the issue like in my diskspace desklet (#937).

With this change, the values are read directly from files using Gio's load_contents_async instead of spawning head or cat subprocesses to read it.

For scenarios where this is not possible (NVIDIA GPU monitoring), I switched subprocess.wait_async to load_contents_async and load_contents_finish which seems to be the correct way to read subprocess output async, as described here in the Gio docs.

[rcalixte]

@rcassani This seems like a noteworthy bug fix so it would be nice to have it merged sooner than later if possible. If you would like some time to review, that's fine too.

[rcassani]

Hi @schorschii, nice work!
It was kind of reported in #913, which is properly solved with this PR.
@rcalixte, ready to be merged

[rcalixte]

Thanks to both of you!