Outline some modules for coming steps

The UI parts will end up in "widgets.rs". Buttons, labels, etc.

The "contsants.rs" module is meant to have compile-time configurables.
Bevy's reflection abilities make it possible to alter these at runtime,
which means this module will actually have the startup defaults rather
than true program constants.

src/constants.rs

@@ -0,0 +1,12 @@
+//! Program constants & defaults
+
+use bevy::prelude::*;
+
+#[derive(Debug, Reflect, Resource)]
+struct ConfigDefaults {}
+
+impl FromWorld for ConfigDefaults {
+  fn from_world(world: &mut World) -> Self {
+    todo!()
+  }
+}

src/main.rs

@@ -2,6 +2,8 @@ use bevy::{prelude::*, window::WindowResolution};
 use bevy_inspector_egui::{bevy_egui::EguiPlugin, quick::WorldInspectorPlugin};
 
 mod card;
+mod constants;
+mod widgets;
 
 fn main() {
   App::new()

src/widgets.rs

@@ -0,0 +1 @@
+//! Catch-all location for UI bits