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/states/skirmish.rs

@@ -1,4 +1,4 @@
-use crate::app::states::skirmish_states::BoardState;
+use crate::{app::states::skirmish_states::BoardState, cli::Cli};
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct SkirmishState {
@@ -6,4 +6,15 @@ pub struct SkirmishState {
     pub name: &'static str,
     pub board: BoardState,
     pub side_panel: bool,
+    pub turn_counter: u64,
+}
+
+impl SkirmishState {
+    pub fn update(&mut self, cli: &Cli) {
+        self.board.advance_turn();
+
+        // if self.board.is_victory() {}
+
+        self.turn_counter += 1;
+    }
 }