improve progress bar, refactor node destruction, simplify layers export

src/node.h

@@ -116,7 +116,6 @@ public:
     bool m_flood_events = false;
     bool m_force_mouse_capture = false;
     bool m_capture_children = true; // wether to capture children events when xx_capture() is used
-    bool m_destroyed = false;
     std::vector<Node*> m_capture_stack;
 
     bool m_mouse_ignore = true;
@@ -201,8 +200,7 @@ public:
     virtual void clone_children(Node* dest) const;
     virtual void clone_finalize(Node* dest) const;;
     void watch(std::function<bool(Node*)> observer);
-    void destroy();
-    virtual void destroy_immediate();
+    virtual void destroy();
     Node* root();
 
     template<class T = Node> T* find(const char* ids)