Add tick thread and Skirmish state update

Introduce `AppEvent::Tick` and a background thread that emits timed
events. `App` now handles `Tick` by calling a new `update` method,
which advances the board and increments a `turn_counter` in
`SkirmishState`. Add required imports, fields, and the
`spawn_tick_thread` call in `main`.

src/app/threads/mod.rs

@@ -1,5 +1,7 @@
 mod audio;
 mod events;
+mod tick;
 
 pub use audio::{AudioCmd, SoundrackParts, Soundtrack, handle_audio};
 pub use events::{AppEvent, handle_events};
+pub use tick::spawn_tick_thread;