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/python/intern/bpy_interface.c

@@ -808,7 +808,7 @@ int BPY_context_member_get(bContext *C, const char *member, bContextDataResult *
     ptr = &(((BPy_StructRNA *)item)->ptr);
 
     // result->ptr = ((BPy_StructRNA *)item)->ptr;
-    CTX_data_pointer_set(result, ptr->id.data, ptr->type, ptr->data);
+    CTX_data_pointer_set(result, ptr->owner_id, ptr->type, ptr->data);
     CTX_data_type_set(result, CTX_DATA_TYPE_POINTER);
     done = true;
   }
@@ -834,7 +834,7 @@ int BPY_context_member_get(bContext *C, const char *member, bContextDataResult *
           BLI_addtail(&result->list, link);
 #endif
           ptr = &(((BPy_StructRNA *)list_item)->ptr);
-          CTX_data_list_add(result, ptr->id.data, ptr->type, ptr->data);
+          CTX_data_list_add(result, ptr->owner_id, ptr->type, ptr->data);
         }
         else {
           CLOG_INFO(BPY_LOG_CONTEXT,