add G_DEFINE_AUTOPTR_CLEANUP_FUNC to support g_autoptr
g_autoptr is available automatically if those structures, GrlMedia, GrlOperationOptions (in this case) are declared with G_DECLARE_* macros. However, for some reasons (GRL_PADDING, or breaking ABI), it is kind of dangerous that they are migrated to G_DECLARE_* without any knowledge. Instead, G_DEFINE_AUTOPTR_CLEANUP_FUNC is added to support g_autoptr.
The commits are not only related to those TODOs, https://gitlab.gnome.org/GNOME/gnome-photos/blob/master/src/photos-flickr-item.c#L150, and gnome-photos#77, but also to support it for newly-written applications.
TODO
- Implement test cases for g_autoptr.
- Add the cleanup macro to other classes.