IDTypeInfo: add .blend file io callbacks

This is part of T76372.
It adds the `blend_write`, `blend_read_data`, `blend_read_lib`
and `blend_read_expand` which correspond to the various
steps when reading and writing .blend files.
Having these callbacks allows us to decentralize the blenloader
code a lot more. This has the affect that code related to any
specific ID type is less scattered.

Reviewers: mont29

Differential Revision: https://developer.blender.org/D8670

source/blender/blenkernel/intern/library.c

@@ -75,6 +75,12 @@ IDTypeInfo IDType_ID_LI = {
     .free_data = library_free_data,
     .make_local = NULL,
     .foreach_id = library_foreach_id,
+    .foreach_cache = NULL,
+
+    .blend_write = NULL,
+    .blend_read_data = NULL,
+    .blend_read_lib = NULL,
+    .blend_read_expand = NULL,
 };
 
 void BKE_library_filepath_set(Main *bmain, Library *lib, const char *filepath)