More helpful Unicode stuff.

More helpful Unicode stuff.
Conversion classes are now:
wxConv_libc (standard mbstowcs/wcstombs)
wxConv_file (usually same as above, but I think some Asian locales
use different encoding systems on the filenames than on the contents
of the files)
wxConv_UTF7 (7-bit Unicode encoding (similar to BASE64), for email etc,
not implemented yet)
wxConv_UTF8 (8-bit Unicode encoding, not implemented yet, just uses
mbstowcs/wcstombs, which just happens to be UTF-8 in glibc2)
wxConv_gdk (gdk_mbstowcs/gdk_wcstombs, not too useful)
wxConv_local (current 8-bit character set, from LANG env var)

wxConv_current: pointer to conversion class. Points by default to
wxConv_local, but it's meant to be able to be redirected by the user
(and wxGTK can point it at wxConv_UTF8 if it manages to set the UTF-8
locale).

mbc_str() returns mb_str(*wxConv_current) for convenience.
I will use mbc_str() in my upcoming wxGTK adaption.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@2163 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775