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/offset.rs

@@ -0,0 +1,48 @@
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct Offset {
+    value: usize,
+    max: usize,
+    value_initiated: bool,
+    max_initiated: bool,
+}
+
+impl Offset {
+    pub fn new() -> Self {
+        Self {
+            value: 0,
+            max: 0,
+            value_initiated: false,
+            max_initiated: false,
+        }
+    }
+
+    pub fn get_value(&self) -> usize {
+        self.value
+    }
+
+    pub fn set_initial_value(&mut self, value: usize) {
+        if self.value_initiated {
+            return;
+        }
+
+        self.value = value;
+        self.value_initiated = true;
+    }
+
+    pub fn set_max(&mut self, max: usize) {
+        if self.max_initiated {
+            return;
+        }
+
+        self.max = max;
+        self.max_initiated = true;
+    }
+
+    pub fn next(&mut self) {
+        self.value = self.value.saturating_add(1).min(self.max);
+    }
+
+    pub fn prev(&mut self) {
+        self.value = self.value.saturating_sub(1).max(0)
+    }
+}