improve progress bar, refactor node destruction, simplify layers export

src/node_panel_grid.cpp

@@ -467,7 +467,7 @@ void NodePanelGrid::bake_uvs()
     );
     while (pb_value < fb.getHeight())
     {
-        pb->m_progress->SetWidthP((float)pb_value / (float)fb.getHeight() * 100.f);
+        pb->set_progress((float)pb_value / (float)fb.getHeight());
         std::this_thread::sleep_for(std::chrono::milliseconds(100));
     }
     worker.join();