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/view.rs

@@ -13,7 +13,7 @@ pub enum View {
     SkillsConfig,
 }
 
-impl Widget for &mut App {
+impl Widget for &mut App<'_> {
     fn render(self, area: Rect, buf: &mut Buffer)
     where
         Self: Sized,