giscanner: Mark gpointer nodes as nullable by default

gpointer parameters and return types should be marked as nullable by
default, unless:
 • also annotated with (type) and not with (nullable); or
 • explicitly annotated with (not nullable).

This introduces the (not nullable) annotation as a direct opposite to
(nullable). In future, (not) could be extended to invert other
annotations.

https://bugzilla.gnome.org/show_bug.cgi?id=729660