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/units/mod.rs

@@ -0,0 +1,3 @@
+pub mod unit;
+
+pub use unit::Unit;

src/app/units/unit.rs

@@ -0,0 +1,2 @@
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Unit {}