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/keybindings/skirmish.rs

@@ -85,7 +85,7 @@ pub fn skirmish_keybindings(app: &mut App, key_event: &KeyEvent) {
             Action::Space => board.toggle_marking(),
             Action::Backspace => board.undo_marked_cell(),
             Action::Delete => {
-                board.marking_cells = false;
+                board.marked_cells.marking_cells = false;
                 board.clear_marked_cells()
             }
             Action::Tab => {