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/mod.rs

@@ -1,6 +1,7 @@
 mod board;
 mod focused_cell;
 mod game_mode;
+mod marked_cells;
 mod offset;
 mod players;
 pub mod structures;
@@ -11,6 +12,7 @@ pub mod zoom_level;
 pub use board::BoardState;
 pub use focused_cell::{FocusedCell, MoveFocusedCell};
 pub use game_mode::GameMode;
+pub use marked_cells::MarkedCells;
 pub use offset::Offset;
 pub use players::Players;
 pub use zoom_level::{CellSizes, ZoomLevel};