Make wxNumValidator::m_value protected rather than private

This is useful for writing overridden TransferXXX() methods in the
derived validator classes, give a motivating example in the docs.

See https://github.com/wxWidgets/wxWidgets/pull/2222

include/wx/valnum.h

@@ -238,6 +238,10 @@ protected:
                                                     : wxString();
     }
 
+    // This member is protected because it can be useful to the derived classes
+    // in their Transfer{From,To}Window() implementations.
+    ValueType * const m_value;
+
 private:
     // Just a helper which is a common part of TransferToWindow() and
     // NormalizeString(): returns string representation of a number honouring
@@ -258,8 +262,6 @@ private:
     }
 
 
-    ValueType * const m_value;
-
     wxDECLARE_NO_ASSIGN_CLASS(wxNumValidator);
 };