new file format with versioning and layer opacity

src/app_dialogs.cpp

@@ -200,7 +200,10 @@ void App::dialog_browse()
                 async_start();
                 title_update();
                 for (auto& i : canvas->m_canvas->m_order)
-                    layers->add_layer(canvas->m_canvas->m_layers[i].m_name.c_str());
+                {
+                    auto* l = layers->add_layer(canvas->m_canvas->m_layers[i].m_name.c_str());
+                    l->m_opacity->m_value.x = canvas->m_canvas->m_layers[i].m_opacity;
+                }
                 async_end();
             });
             dialog->destroy();