Add Building and Unit structs, extend CellWidget

Introduce empty Building and Unit types and expose them
through new modules. Extend CellWidget to store optional
Unit and Building, update its constructor, and adjust board
rendering to create cells with these new fields (currently always None).

src/app/mod.rs

@@ -1,10 +1,12 @@
 pub mod app;
+pub mod buildings;
 pub mod helpers;
 pub mod keybind;
 pub mod keybindings;
 pub mod state;
 pub mod states;
 pub mod threads;
+pub mod units;
 pub mod view;
 pub mod views;
 pub mod widgets;