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

lib/Slic3r.pm

@@ -167,6 +167,8 @@ sub thread_cleanup {
     *Slic3r::GUI::PresetHints::DESTROY      = sub {};
     *Slic3r::GUI::TabIface::DESTROY         = sub {};
     *Slic3r::OctoPrint::DESTROY             = sub {};
+    *Slic3r::Duet::DESTROY                  = sub {};
+    *Slic3r::PrintHostFactory::DESTROY                  = sub {};
     *Slic3r::PresetUpdater::DESTROY         = sub {};
     return undef;  # this prevents a "Scalars leaked" warning
 }