Trigger background processing update when switching to a preview tab.

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

src/slic3r/GUI/BackgroundSlicingProcess.hpp

@@ -110,6 +110,11 @@ public:
 	State 	state() 	const { return m_state; }
 	bool    idle() 		const { return m_state == STATE_IDLE; }
 	bool    running() 	const { return m_state == STATE_STARTED || m_state == STATE_RUNNING || m_state == STATE_FINISHED || m_state == STATE_CANCELED; }
+    // Returns true if the last step of the active print was finished with success.
+    // The "finished" flag is reset by the apply() method, if it changes the state of the print.
+    // This "finished" flag does not account for the final export of the output file (.gcode or zipped PNGs),
+    // and it does not account for the OctoPrint scheduling.
+    bool    finished() const { return m_print->finished(); }
 
 private:
 	void 	thread_proc();