add Exit trait to modals

src/core.rs

@@ -41,6 +41,14 @@ pub trait Component: Render + Handle + Sync {
     }
 }
 
+pub trait Exit: Component {
+    fn exited (&self) -> bool;
+    fn exit (&mut self);
+    fn boxed (self) -> Box<dyn Exit> where Self: Sized + 'static {
+        Box::new(self)
+    }
+}
+
 /// Anything that implements `Render` + `Handle` can be used as a UI component.
 impl<T: Render + Handle + Sync> Component for T {}