1. changed all "wxMBConv& conv" parameters to "const wxMBConv&"

2. this allows to use wxConvAuto() instead of wxConvUTF8 as default value
   for this parameter in the classes which read text from the file: wxConvAuto
   automatically recognizes the BOM at the start of file and uses the correct
   conversion
3. don't use Windows for UTF-7 conversions as there is no way to make it
   fail on invalid UTF-7 strings; use our own wxMBConvUtf7 instead


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38570 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/file.h

@@ -97,7 +97,7 @@ public:
     // returns the number of bytes written
   size_t Write(const void *pBuf, size_t nCount);
     // returns true on success
-  bool Write(const wxString& s, wxMBConv& conv = wxConvUTF8)
+  bool Write(const wxString& s, const wxMBConv& conv = wxConvUTF8)
   {
       const wxWX2MBbuf buf = s.mb_str(conv);
       size_t size = strlen(buf);
@@ -172,7 +172,7 @@ public:
 
   // I/O (both functions return true on success, false on failure)
   bool Write(const void *p, size_t n) { return m_file.Write(p, n) == n; }
-  bool Write(const wxString& str, wxMBConv& conv = wxConvUTF8)
+  bool Write(const wxString& str, const wxMBConv& conv = wxConvUTF8)
     { return m_file.Write(str, conv); }
 
   // different ways to close the file