Add copy ctor for wxUniCharRef

It just does the default, but C++11 deprecates having copy assignment operator
without copy ctor. Eliminates many GCC -Wdeprecated-copy warnings.

include/wx/unichar.h

@@ -267,6 +267,12 @@ public:
     wxUniCharRef& operator=(const wxUniCharRef& c)
         { if (&c != this) *this = c.UniChar(); return *this; }
 
+#if wxUSE_UNICODE_UTF8
+    wxUniCharRef(const wxUniCharRef& that) : m_str(that.m_str), m_pos(that.m_pos) { }
+#else
+    wxUniCharRef(const wxUniCharRef& that) : m_pos(that.m_pos) { }
+#endif
+
 #define wxUNICHAR_REF_DEFINE_OPERATOR_EQUAL(type) \
     wxUniCharRef& operator=(type c) { return *this = wxUniChar(c); }
     wxDO_FOR_CHAR_INT_TYPES(wxUNICHAR_REF_DEFINE_OPERATOR_EQUAL)