change node children from unique to shared ptr, rename Canvas2D to StrokePreview, add panel toolbar with icon buttons to toggle

engine/app.h

@@ -20,10 +20,12 @@ public:
     NodePopupMenu* menu_edit = nullptr;
     NodePopupMenu* menu_layers = nullptr;
     NodeBorder* sidebar = nullptr;
-    NodePanelBrush* brushes;
-    NodePanelLayer* layers;
-    NodePanelColor* color;
-    NodePanelStroke* stroke;
+    std::shared_ptr<NodePanelBrush> brushes;
+    std::shared_ptr<NodePanelLayer> layers;
+    std::shared_ptr<NodePanelColor> color;
+    std::shared_ptr<NodePanelStroke> stroke;
+    Node* current_panel = nullptr;
+    Node* panels;
     std::function<void(int)> on_brush_select;
     std::function<void(glm::vec4 color)> on_color_change;
     std::function<void()> on_stroke_change;