Track window area and refactor keybinding APIs

- Track terminal size in `App::window_area` and update on resize
- Accept action slices in `count_largest_group` and
  `KeybindingsWidget::new`
- Add ACTIONS slices and layout helpers for default, main_menu, skirmish
- Add `main_menu_option_helper` for formatting menu strings and export
  it

src/app/keybindings/keybindings.rs

@@ -284,7 +284,7 @@ pub fn event_to_action(event: &KeyEvent) -> Option<Action> {
 /// It counts how many times each group appears in the supplied actions and
 /// returns the highest count. This can be used, for example, to size UI
 /// elements that display groups of bindings.
-pub fn count_largest_group(actions: &Vec<Action>) -> u16 {
+pub fn count_largest_group(actions: &[Action]) -> u16 {
     let mut group_counts: HashMap<Group, u16> = HashMap::new();
 
     for action in actions {