Implemented disabling of Add buttons if next layer range couldn't be added

+ Fixed "Layer ranges" expand.

src/slic3r/GUI/GUI_ObjectList.hpp

@@ -327,6 +327,7 @@ public:
     // may have been postponed from the "kill focus" event of a text field, if the focus was lost for the "add layer" button.
     // Rather providing the range by a value than by a reference, so that the memory referenced cannot be invalidated.
     void add_layer_range_after_current(const t_layer_height_range current_range);
+    wxString can_add_new_range_after_current( t_layer_height_range current_range);
     void add_layer_item (const t_layer_height_range& range, 
                          const wxDataViewItem layers_item, 
                          const int layer_idx = -1);