Add an analyser for GObject nullability
Add an analyser which assumes (obj != NULL) for all NSPACE_IS_OBJ(x) checks, to avoid the current popular false positive from the analyser, where it sees the (obj != NULL) check in the macro expansion and assumes that means the object may be NULL.
This may introduce false negatives if no other non-NULL checks are performed on an object variable which may legitimately be NULL (but such checks would have to be performed, to differentiate the cases (obj = NULL) and (obj != NULL && !NSPACE_IS_OBJ(obj))). This should eliminate a lot of false positives.
(Imported from bugs-everywhere; see #2 (closed).)