Trigger background processing update when switching to a preview tab.

Implements "Go Direct to the preview screen after slicing #152"

src/libslic3r/Print.hpp

@@ -297,7 +297,10 @@ public:
 
     // methods for handling state
     bool                is_step_done(PrintStep step) const { return Inherited::is_step_done(step); }
+    // Returns true if an object step is done on all objects and there's at least one object.    
     bool                is_step_done(PrintObjectStep step) const;
+    // Returns true if the last step was finished with success.
+    bool                finished() const override { return this->is_step_done(psGCodeExport); }
 
     bool                has_infinite_skirt() const;
     bool                has_skirt() const;