RNA: Cleanup PointerRNA struct

The old layout of `PointerRNA` was confusing for historic reasons:
```
typedef struct PointerRNA {
  struct {
    void *data;
  } id;

  struct StructRNA *type;
  void *data;
} PointerRNA;
```

This patch updates it to:
```
typedef struct PointerRNA {
  struct ID *owner_id;
  struct StructRNA *type;
  void *data;
} PointerRNA;
```

Throughout the code base `id.data` was replaced with `owner_id`.
Furthermore, many explicit pointer type casts were added which
were implicit before. Some type casts to `ID *` were removed.

Reviewers: brecht, campbellbarton

Differential Revision: https://developer.blender.org/D5558

source/blender/editors/object/object_hook.c

@@ -436,7 +436,7 @@ static void object_hook_from_context(
   HookModifierData *hmd;
 
   if (ptr->data) { /* if modifier context is available, use that */
-    ob = ptr->id.data;
+    ob = (Object *)ptr->owner_id;
     hmd = ptr->data;
   }
   else { /* use the provided property */