Unicode handling:

Removed the Perl dependencies on Encode, Encode::Locale and Unicode::Normalize.
Added dependency on boost::locale.
Added encode_path, decode_path, normalize_utf8 functions to Slic3r.xs

Slic3r.xs has been made mostly utf8 safe by using the boost::nowide library,
thanks to @alexrj for the idea.

Simplified the encode_path / decode_path stuff:
wxWidgets are unicode already, so there is no need to decode_path() from it.
Perl / win32 interfacing is non-unicode, so decode_path() is executed
on ARGV just at the beginning of the perl scripts.

xs/xsp/XS.xsp

@@ -48,6 +48,24 @@ trace(level, message)
     CODE:
         Slic3r::trace(level, message);
 
+std::string
+encode_path(src)
+    const char *src;
+    CODE:
+        Slic3r::encode_path(src);
+
+std::string
+decode_path(src)
+    const char *src;
+    CODE:
+        Slic3r::decode_path(src);
+
+std::string
+normalize_utf8_nfc(src)
+    const char *src;
+    CODE:
+        Slic3r::normalize_utf8_nfc(src);
+
 void
 xspp_test_croak_hangs_on_strawberry()
     CODE: