Centralize legacy animation bridge

src/node_panel_animation.h

@@ -9,6 +9,10 @@
 
 class Layer;
 
+namespace pp::panopainter {
+class LegacyDocumentAnimationServices;
+}
+
 namespace pp::app {
 struct DocumentAnimationPanelState;
 struct DocumentAnimationOperationPlan;
@@ -52,6 +56,8 @@ public:
 
 class NodePanelAnimation : public Node
 {
+    friend class pp::panopainter::LegacyDocumentAnimationServices;
+
     NodeButtonCustom* btn_next = nullptr;
     NodeButtonCustom* btn_prev = nullptr;
     NodeButtonCustom* btn_play = nullptr;