Switching to the preview screen when the "Slice now" button is pressed.

Let's wait for the response of the beta testers whether it is a good
idea.

src/slic3r/GUI/Plater.cpp

@@ -747,6 +747,7 @@ Sidebar::Sidebar(Plater *parent)
             p->plater->export_gcode();
         else
             p->plater->reslice();
+		p->plater->select_view_3D("Preview");
     });
     p->btn_send_gcode->Bind(wxEVT_BUTTON, [this](wxCommandEvent&) { p->plater->send_gcode(); });
 }