This updates demos to the point where they can run without
warnings. Some of them become less clear, this is addressed
(where I noticed it) in follow-up commits.
Fixes#3209
I think it's clearer if only the actual demos started with `demo_`,
so I renamed `demo_helpers.sh` to just `helpers.sh`.
`demo_resolve_conflict.sh` should match `resolve_conflicts.png` (with an s).
The demos don't need to be animated - the user wouldn't miss anything
if they skipped to the end. So let's just show the full output so the
user can read through it at their own pace. We could use plain text,
but I think the colors are helpful, so I went with screenshots.
Closes#166.
