gsettings: Add various missing (nullable) or (not nullable) annotations

Also added (transfer full) or (transfer none) annotations while I was at it.

This is the result of checking each `Returns:` line in these files. I’ve
only considered nullability and transferability, and not other (potentially
missing or incorrect) annotations.

Helps: #2227
10 jobs for gio-add-gsettings-nullable-annotations in 12 minutes and 15 seconds (queued for 6 seconds)