Fixed "Slic3r crashes when sending STLs with special characters to the printer"

https://github.com/prusa3d/Slic3r/issues/597
The "Send to OctoPrint" function will now send the file name encoded
in UTF-8, so the file name will not get mangled.

The C++ Slic3r::encode_path() function was returning a string to Perl,
which was marked as UTF-8. This has been fixed, now encode_path() returns
a plain Perl string.

Added path_to_filename, path_to_stem, path_to_extension, path_to_parent_path
Perl wrappers to boost::filesystem::path splitting functionality
to be able to split UTF-8 encoded files on Windows correctly.

xs/xsp/XS.xsp

@@ -91,7 +91,7 @@ data_dir()
         RETVAL = const_cast<char*>(Slic3r::data_dir().c_str());
     OUTPUT: RETVAL
 
-std::string
+local_encoded_string
 encode_path(src)
     const char *src;
     CODE:
@@ -112,6 +112,34 @@ normalize_utf8_nfc(src)
         RETVAL = Slic3r::normalize_utf8_nfc(src);
     OUTPUT: RETVAL
 
+std::string
+path_to_filename(src)
+    const char *src;
+    CODE:
+        RETVAL = Slic3r::PerlUtils::path_to_filename(src);
+    OUTPUT: RETVAL
+
+std::string
+path_to_stem(src)
+    const char *src;
+    CODE:
+        RETVAL = Slic3r::PerlUtils::path_to_stem(src);
+    OUTPUT: RETVAL
+
+std::string
+path_to_extension(src)
+    const char *src;
+    CODE:
+        RETVAL = Slic3r::PerlUtils::path_to_extension(src);
+    OUTPUT: RETVAL
+
+std::string
+path_to_parent_path(src)
+    const char *src;
+    CODE:
+        RETVAL = Slic3r::PerlUtils::path_to_parent_path(src);
+    OUTPUT: RETVAL
+
 void
 xspp_test_croak_hangs_on_strawberry()
     CODE: