improve progress bar, refactor node destruction, simplify layers export

src/node_progress_bar.h

@@ -11,6 +11,11 @@ public:
     NodeButton* btn_cancel;
     NodeText* m_title;
     NodeBorder* m_progress;
+    int m_total = 0;
+    int m_count = 0;
     virtual Node* clone_instantiate() const override;
     virtual void init() override;
+    void increment() noexcept;
+    // set progress where p [0, 1]
+    void set_progress(float p) noexcept;
 };