introduce View and Previews traits

In this commit we refactor the preview mechanism, and switch to
responsive views by default.

To create a preview, your view now has to implement the Preview trait.
This is very similar to SwiftUI's preview mechanism.

Signed-off-by: William Casarin <jb55@jb55.com>

src/ui/preview.rs

@@ -0,0 +1,33 @@
+use crate::ui::View;
+
+pub trait Preview {
+    type Prev: View;
+
+    fn preview() -> Self::Prev;
+}
+
+pub struct PreviewApp {
+    view: Box<dyn View>,
+}
+
+impl<V> From<V> for PreviewApp
+where
+    V: View + 'static,
+{
+    fn from(v: V) -> Self {
+        PreviewApp::new(v)
+    }
+}
+
+impl PreviewApp {
+    pub fn new(view: impl View + 'static) -> PreviewApp {
+        let view = Box::new(view);
+        Self { view }
+    }
+}
+
+impl eframe::App for PreviewApp {
+    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
+        egui::CentralPanel::default().show(ctx, |ui| self.view.ui(ui));
+    }
+}