Centralize legacy brush UI bridge

src/node_panel_brush.h

@@ -12,6 +12,9 @@
 namespace pp::app {
 struct BrushTextureListPlan;
 }
+namespace pp::panopainter {
+class LegacyBrushTextureListServices;
+}
 
 class NodeButtonBrush : public NodeButtonCustom, public Serializer::Type
 {
@@ -34,6 +37,8 @@ public:
 
 class NodePanelBrush : public Node
 {
+    friend class pp::panopainter::LegacyBrushTextureListServices;
+
     // brushes that are marked as deleted but file still exists
     std::vector<std::string> m_deleted;
     NodeButtonBrush* m_current = nullptr;