Add support for UTF8 filenames in wxZipInputstream.

Zip files may mark filenames and comments via bit 11 in the general purpose
flags. If this flag is set use wxConvUTF8 instead of the default wxConv.

include/wx/zipstrm.h

@@ -105,6 +105,7 @@ enum wxZipFlags
     wxZIP_ENHANCED          = 0x0010,
     wxZIP_PATCH             = 0x0020,
     wxZIP_STRONG_ENC        = 0x0040,
+    wxZIP_LANG_ENC_UTF8     = 0x0800,   // filename and comment are UTF8
     wxZIP_UNUSED            = 0x0F80,
     wxZIP_RESERVED          = 0xF000
 };