Refactor ruin_app, add some README files, minor usability in reactivity

lib/reactivity/examples/readme_md_03.rs

@@ -0,0 +1,29 @@
+use std::{cell::Cell, rc::Rc, time::Duration};
+
+use ruin_reactivity::event;
+use ruin_runtime::{main, queue_future, set_interval, time::sleep};
+
+#[main]
+fn main() {
+    let my_event = event::<String>();
+
+    my_event.subscribe(|message| {
+        println!("got event with message: {message}");
+    });
+
+    // Emit an event every 250ms with an incrementing count.
+    let interval = set_interval(Duration::from_millis(250), {
+        let counter = Rc::new(Cell::new(0));
+        move || {
+            let count = counter.get();
+            my_event.emit(format!("the count is {}", count));
+            counter.set(count + 1);
+        }
+    });
+
+    // After 5 seconds, clear the interval to stop emitting events.
+    queue_future(async move {
+        sleep(Duration::from_secs(5)).await;
+        interval.clear();
+    });
+}