Plain Old Data (POD) types with or without a representation in the GType
type system can still have a copy and/or a free function. We should
allow annotating these types with their corresponding functions for
copying their data into a new instance, and freeing their data.

From a language bindings perspective, POD types should have a boxed
GType wrapper around them, so they can use the generic GBoxed API to
copy and free instances; from a documentation perspective, though, it'd
be good to have a way to match a structured type, like a struct or a
union, with its copy and free functions.

In order to do that, we add two new header block annotations:

- (copy-func function_name)
- (free-func function_name)

These annotations work exactly like ref-func and unref-func for typed
instances:

    /**
     * GdkRGBA: (copy-func gdk_rgba_copy)
     *   (free-func gdk_rgba_free)
     * @ReD: ...
     * @green: ...
     * @blue: ...
     * @alpha: ...
     *
     *...