separate global tick from draw so on tick any node can request a redraw

src/app_events.cpp

@@ -11,6 +11,11 @@ std::string win32_open_file();
 
 using namespace ui;
 
+void App::tick(float dt)
+{
+    layout[main_id]->tick(dt);
+}
+
 void App::resize(float w, float h)
 {
     redraw = true;