redo event fn signature & add event_ctx macro

examples/minimal.rs

@@ -1,20 +1,14 @@
 use iris::{prelude::*, winit::*};
 
 fn main() {
-    UiApp::<State>::run();
+    DefaultApp::<State>::run();
 }
 
-struct State {
-    ui: DefaultUi,
-}
+struct State;
 
-impl DefaultUiState for State {
-    fn new(mut ui: DefaultUi, _proxy: Proxy<Self::Event>) -> Self {
-        rect(Color::RED).set_root(&mut ui);
-        Self { ui }
-    }
-
-    fn ui(&mut self) -> &mut DefaultUi {
-        &mut self.ui
+impl DefaultAppState for State {
+    fn new(ui: &mut Ui, _state: &UiState, _proxy: Proxy<Self::Event>) -> Self {
+        rect(Color::RED).set_root(ui);
+        Self
     }
 }