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

build/bakefiles/files.bkl

@@ -290,6 +290,7 @@ IMPORTANT: please read docs/tech/tn0016.txt before modifying this file!
     src/common/clntdata.cpp
     src/common/cmdline.cpp
     src/common/config.cpp
+    src/common/convauto.cpp
     src/common/datetime.cpp
     src/common/datstrm.cpp
     src/common/dircmn.cpp
@@ -362,6 +363,7 @@ IMPORTANT: please read docs/tech/tn0016.txt before modifying this file!
     wx/cmdline.h
     wx/confbase.h
     wx/config.h
+    wx/convauto.h
     wx/containr.h
     wx/datetime.h
     wx/datstrm.h