QList: add data_ptr() methods like the ones in QString/QBA

It will be used in a suqsequent commit (adding QString::{to,from}Utf32).

Change-Id: Ie113a47f0ff0e9a7b26a6d0f47e17d24a4a88465
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qlist.h

@@ -107,6 +107,9 @@ public:
     using rvalue_ref = T &&;
 #endif
 
+    DataPointer &data_ptr() { return d; }
+    const DataPointer &data_ptr() const { return d; }
+
     class const_iterator;
     class iterator {
         friend class QList<T>;

src/corelib/tools/qlist.qdoc

@@ -1638,3 +1638,11 @@
     This function only allocates memory if the number of elements in \a l
     exceeds the capacity of this list or this list is shared.
 */
+
+/*!
+    \fn template <typename T> QList<T>::DataPointer &QList<T>::data_ptr();
+    \fn template <typename T> const QList<T>::DataPointer &QList<T>::data_ptr() const;
+
+    \internal
+    \since 6.10
+*/