Move skirmish Offset and FocusedCell to own module

Extract Offset and FocusedCell structs from skirmish.rs into a new
skirmish_states module. Re-export the structs from the top-level states
module and adjust imports in skirmish.rs.

src/app/states/skirmish_states/mod.rs

@@ -0,0 +1,5 @@
+pub mod focused_cell;
+pub mod offset;
+
+pub use focused_cell::FocusedCell;
+pub use offset::Offset;