(pomodoro/countdown) Change initial value (#79)

- While editing, an user can apply changes as a new initial value (pomodoro/countdown only).
- New keybinding: `[^s]save initial value` 
- Update keybinding: `[s]ave changes` (instead of `[Enter]`)
- Refactor event handling to re-structure `edit` / `non-edit` modes.
- Refactor footer to reflect latest keybindings

src/widgets/clock.rs

@@ -143,6 +143,10 @@ impl<T> ClockState<T> {
         &self.initial_value
     }
 
+    pub fn set_initial_value(&mut self, duration: DurationEx) {
+        self.initial_value = duration;
+    }
+
     pub fn get_current_value(&self) -> &DurationEx {
         &self.current_value
     }