improve layer history, fix doc open

src/node_panel_layer.h

@@ -74,10 +74,27 @@ public:
     void handle_layer_highlight(NodeLayer* target, bool highlight);
     void handle_layer_blend_mode(NodeLayer* target, int mode);
     void handle_layer_selected(NodeLayer* target);
+    void save_history();
     void clear();
     void update_attributes();
 };
 
+struct ActionLayerChange : public Action
+{
+    struct LayerState
+    {
+        bool visible;
+        float alpha;
+        bool lock;
+    };
+    NodePanelLayer* m_panel;
+    std::vector<LayerState> m_layers;
+    virtual void run() override { }
+    virtual Action* get_redo() override;
+    virtual void undo() override;
+    virtual size_t memory() override { return 0; }
+};
+
 class ActionLayerAdd : public Action
 {
 public: