Created attachment 276109 [details] [review]
introspection: add many missing Returns: (nullable) annotations

Now that bug #660879 has been fixed, I'm trying to go through and add the missing annotations.

I've never used ATK, so I'm not too familiar with the API, but I've gone through all of the Returns tags and, where there is text mentioning a potential NULL return value, I've added a (nullable) annotation.  I'm sure there are other functions which may return NULL but I doubt I have added annotations where they aren't appropriate, so this should be safe.