Removes bottom status bar.

StatusBar class calls are commented out and replaced with notifications.
SlicicingProgress notification shows progress of slicing, ProgressIndicator notification handles other progress information, like arrange objects etc.

src/slic3r/GUI/GUI_App.hpp

@@ -276,7 +276,7 @@ public:
     ObjectLayers*        obj_layers();
     Plater*              plater();
     Model&      		 model();
-    NotificationManager* notification_manager();
+    std::shared_ptr<NotificationManager> notification_manager();
 
     // Parameters extracted from the command line to be passed to GUI after initialization.
     GUI_InitParams* init_params { nullptr };