Add GameStates, typed enums, and App::new constructor

src/app/mod.rs

@@ -1,6 +1,10 @@
 pub mod app;
+pub mod game_states;
 pub mod keybindings;
 pub mod widget;
 pub mod widgets;
 
-pub use app::{App, Event, View, handle_input_events};
+pub use app::{App, Event, GameMode, PerkDecks, View, handle_input_events};
+pub use game_states::{
+    GameStates, MainMenuState, PerkDecksState, SettingsState, SkillsConfigState, SkirmishState,
+};