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/interface/interface_utils.c

@@ -405,7 +405,7 @@ void ui_rna_collection_search_cb(const struct bContext *C,
   RNA_PROP_BEGIN (&data->search_ptr, itemptr, data->search_prop) {
 
     if (flag & PROP_ID_SELF_CHECK) {
-      if (itemptr.data == data->target_ptr.id.data) {
+      if (itemptr.data == data->target_ptr.owner_id) {
         continue;
       }
     }
@@ -566,7 +566,7 @@ int UI_calc_float_precision(int prec, double value)
 
 bool UI_but_online_manual_id(const uiBut *but, char *r_str, size_t maxlength)
 {
-  if (but->rnapoin.id.data && but->rnapoin.data && but->rnaprop) {
+  if (but->rnapoin.owner_id && but->rnapoin.data && but->rnaprop) {
     BLI_snprintf(r_str,
                  maxlength,
                  "%s.%s",