Fixing build against wxWidgets 3.0

The wxString saga continues. wxWidgets 3.0 don't have the wxString::FromUTF8(const std::string&) overload, we must use the GUI::from_u8 helper
Also wxWidgets 3.0 don't allow to disable wxString->const char* conversion, so calling show_info(wxWindow*, wxString, const char*) was ambiguous
Several includes moved around

src/libslic3r/Config.hpp

@@ -16,8 +16,8 @@
 #include "Point.hpp"
 
 #include <boost/algorithm/string/trim.hpp>
-#include <boost/format.hpp>
-#include <boost/property_tree/ptree.hpp>
+#include <boost/format/format_fwd.hpp>
+#include <boost/property_tree/ptree_fwd.hpp>
 
 #include <cereal/access.hpp>
 #include <cereal/types/base_class.hpp>

src/libslic3r/Extruder.cpp

@@ -1,4 +1,5 @@
 #include "Extruder.hpp"
+#include "PrintConfig.hpp"
 
 namespace Slic3r {
 

src/libslic3r/Extruder.hpp

@@ -3,10 +3,11 @@
 
 #include "libslic3r.h"
 #include "Point.hpp"
-#include "PrintConfig.hpp"
 
 namespace Slic3r {
 
+class GCodeConfig;
+
 class Extruder
 {
 public: