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/mod.rs

@@ -1,11 +1,17 @@
 pub mod note;
+pub mod preview;
 pub mod username;
 
 pub use note::Note;
+pub use preview::{Preview, PreviewApp};
 pub use username::Username;
 
 use egui::Margin;
 
+pub trait View {
+    fn ui(&mut self, ui: &mut egui::Ui);
+}
+
 pub fn padding<R>(
     amount: impl Into<Margin>,
     ui: &mut egui::Ui,
@@ -15,3 +21,9 @@ pub fn padding<R>(
         .inner_margin(amount)
         .show(ui, add_contents)
 }
+
+pub fn is_mobile(ctx: &egui::Context) -> bool {
+    //true
+    let screen_size = ctx.screen_rect().size();
+    screen_size.x < 550.0
+}