Remove some uses of top-level panels in our examples (#7729)

We're phasing out top-level panels (panels that use `Context` directly,
instead of being inside another `Ui`).
As a first step, stop using them in our demo library and application.

* Part of https://github.com/emilk/egui/issues/3524

crates/egui_demo_app/src/lib.rs

@@ -15,6 +15,14 @@ pub(crate) fn seconds_since_midnight() -> f64 {
     time.num_seconds_from_midnight() as f64 + 1e-9 * (time.nanosecond() as f64)
 }
 
+/// Trait that wraps different parts of the demo app.
+pub trait DemoApp {
+    fn demo_ui(&mut self, ui: &mut egui::Ui, frame: &mut eframe::Frame);
+
+    #[cfg(feature = "glow")]
+    fn on_exit(&mut self, _gl: Option<&eframe::glow::Context>) {}
+}
+
 // ----------------------------------------------------------------------------
 #[cfg(feature = "accessibility_inspector")]
 pub mod accessibility_inspector;