Added possibility for upload to Duet

Further changes:
- Added new configuration option Host Type
- Added abstract base class for future printer hosts
- Moved location of upload dialog (also made it a little bit more configureable)
- added possibility to send file via postfield instead a new frame

xs/xsp/Utils_PrintHost.xsp

@@ -0,0 +1,10 @@
+%module{Slic3r::XS};
+
+%{
+#include <xsinit.h>
+#include "slic3r/Utils/PrintHost.hpp"
+%}
+
+%name{Slic3r::PrintHost} class PrintHost {
+	bool send_gcode(std::string filename) const;
+};