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.

Build.PL

@@ -8,8 +8,6 @@ use File::Spec;
 
 my %prereqs = qw(
     Devel::CheckLib                 0
-    Encode                          0
-    Encode::Locale                  1.05
     ExtUtils::MakeMaker             6.80
     ExtUtils::ParseXS               3.22
     File::Basename                  0
@@ -24,7 +22,6 @@ my %prereqs = qw(
     IO::Scalar                      0
     threads                         1.96
     Time::HiRes                     0
-    Unicode::Normalize              0
 );
 my %recommends = qw(
     Class::XSAccessor               0