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/app.rs

@@ -6,6 +6,7 @@ use crate::imgcache::ImageCache;
 use crate::notecache::NoteCache;
 use crate::timeline;
 use crate::ui;
+use crate::ui::is_mobile;
 use crate::Result;
 use egui::containers::scroll_area::ScrollBarVisibility;
 
@@ -87,12 +88,6 @@ pub struct Damus {
     frame_history: crate::frame_history::FrameHistory,
 }
 
-pub fn is_mobile(ctx: &egui::Context) -> bool {
-    //true
-    let screen_size = ctx.screen_rect().size();
-    screen_size.x < 550.0
-}
-
 fn relay_setup(pool: &mut RelayPool, ctx: &egui::Context) {
     let ctx = ctx.clone();
     let wakeup = move || {