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_states/marked_cells.rs

@@ -0,0 +1,19 @@
+use crate::app::states::skirmish_states::units::Units;
+use std::collections::VecDeque;
+
+#[derive(Debug, Clone, PartialEq)]
+pub struct MarkedCells<'a> {
+    pub marking_cells: bool,
+    pub marked_cells: VecDeque<(usize, usize)>,
+    pub selected_unit: &'a Option<Units>,
+}
+
+impl MarkedCells<'_> {
+    pub fn new() -> Self {
+        Self {
+            marking_cells: false,
+            marked_cells: VecDeque::new(),
+            selected_unit: &None,
+        }
+    }
+}