Add C++11-style cbegin, cend, etc. to wxString's iterator interface

These c-prefixed functions always return a const_iterator.
The full list of functions added: cbegin(), cend(), crbegin(), crend().

Closes https://github.com/wxWidgets/wxWidgets/pull/314

docs/changes.txt

@@ -74,6 +74,7 @@ All:
 - Add support for loading dynamic lexer in wxStyledTextCtrl (New Pagodi).
 - Handle strings with embedded NULs in wxDataStream (Nitch).
 - Don't crash in wxTextFile::GetLastLine() if the file is empty (crohr).
+- Add wxString::cbegin() and cend() method (Lauri Nurmi).
 
 All (GUI):
 

include/wx/string.h

@@ -1282,20 +1282,26 @@ public:
   // first valid index position
   const_iterator begin() const { return const_iterator(this, m_impl.begin()); }
   iterator begin() { return iterator(this, m_impl.begin()); }
+  const_iterator cbegin() const { return const_iterator(this, m_impl.begin()); }
   // position one after the last valid one
   const_iterator end() const { return const_iterator(this, m_impl.end()); }
   iterator end() { return iterator(this, m_impl.end()); }
+  const_iterator cend() const { return const_iterator(this, m_impl.end()); }
 
   // first element of the reversed string
   const_reverse_iterator rbegin() const
     { return const_reverse_iterator(end()); }
   reverse_iterator rbegin()
     { return reverse_iterator(end()); }
+  const_reverse_iterator crbegin() const
+    { return const_reverse_iterator(end()); }
   // one beyond the end of the reversed string
   const_reverse_iterator rend() const
     { return const_reverse_iterator(begin()); }
   reverse_iterator rend()
     { return reverse_iterator(begin()); }
+  const_reverse_iterator crend() const
+    { return const_reverse_iterator(begin()); }
 
   // std::string methods:
 #if wxUSE_UNICODE_UTF8

interface/wx/string.h

@@ -1574,13 +1574,17 @@ public:
 
     const_iterator begin() const;
     iterator begin();
+    const_iterator cbegin() const;
     const_iterator end() const;
     iterator end();
+    const_iterator cend() const;
 
     const_reverse_iterator rbegin() const;
     reverse_iterator rbegin();
+    const_reverse_iterator crbegin() const;
     const_reverse_iterator rend() const;
     reverse_iterator rend();
+    const_reverse_iterator crend() const;
 
     //@}