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/state.rs

@@ -35,6 +35,7 @@ impl GameStates {
                     false,
                 ),
                 side_panel: false,
+                turn_counter: 0,
             },
             perk_decks: PerkDecksState {
                 id: 2,