Add lifetime parameter and MarkedCells struct

Refactor App, GameStates, SkirmishState and BoardState to carry a
generic
lifetime `'a`. Introduce a new `MarkedCells` type that holds marking
state
and selected unit, replacing the previous `marking_cells` flag and raw
deque. Update related methods, keybindings, view implementations and
module exports to use the new structures.

src/app/states/skirmish.rs

@@ -1,15 +1,15 @@
 use crate::app::states::skirmish_states::BoardState;
 
 #[derive(Debug, Clone, PartialEq)]
-pub struct SkirmishState {
+pub struct SkirmishState<'a> {
     pub id: usize,
     pub name: &'static str,
-    pub board: BoardState,
+    pub board: BoardState<'a>,
     pub side_panel: bool,
     pub turn_counter: u64,
 }
 
-impl SkirmishState {
+impl SkirmishState<'_> {
     pub fn tick_update(&mut self) {
         // self.board.advance_turn();