UnsavedChangesDialog : improvements

* Processed changes in options with nullable values
* Processed changes on the extruders count

src/slic3r/GUI/Tab.hpp

@@ -411,6 +411,7 @@ public:
 	size_t		m_extruders_count_old = 0;
 	size_t		m_initial_extruders_count;
 	size_t		m_sys_extruders_count;
+	size_t		m_cache_extruder_count = 0;
 
     PrinterTechnology               m_printer_technology = ptFFF;
 
@@ -437,6 +438,8 @@ public:
     bool 		supports_printer_technology(const PrinterTechnology /* tech */) override { return true; }
 
 	wxSizer*	create_bed_shape_widget(wxWindow* parent);
+	void		cache_extruder_cnt();
+	void		apply_extruder_cnt_from_cache();
 };
 
 class TabSLAMaterial : public Tab