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_creation.rs

@@ -1,6 +1,6 @@
-use crate::app::is_mobile;
 use crate::app_style::{create_text_styles, dark_mode, desktop_font_size, mobile_font_size};
 use crate::fonts::setup_fonts;
+use crate::ui::is_mobile;
 use eframe::NativeOptions;
 
 pub const UI_SCALE_FACTOR: f32 = 0.2;