Removal of pointless MFC dependency
Fix for garbage data in XRef file names in "unicode" builds.
1.  Read of released char array has been fixed.
2.  toUTF8String call in related line has been replaced
    by wideString2utf8String as, at least on MSVC11, std::stringstream lacks
    wchat_t* overload for << operator which causes wchars to be written
    as series of int16 values.... same problem seems to be prevalent
    in less often used (and tested) code.
Inappropriate use of toUTF8String has been fixed in few more places.
XRef export improvements.
1.  XRef scenes are now exported as <instance_node> elements either
    in standalone <node> element with name "XRefs" or in node corresponding
    to scene element to which XRefed file is bound.
2.  "Include XRefs" option in export dialog is now visible and actually
    works. It controls whether referenced scenes are also exported
    - not everyone needs this feature and it can considerably extend
    export times...