Move code from egui_demo_lib to egui_demo_app (#1540)

Also clean up feature names and dependencies

egui_demo_app/src/main.rs

@@ -16,6 +16,6 @@ fn main() {
     eframe::run_native(
         "egui demo app",
         options,
-        Box::new(|cc| Box::new(egui_demo_lib::WrapApp::new(cc))),
+        Box::new(|cc| Box::new(egui_demo_app::WrapApp::new(cc))),
     );
 }