(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/app.rs

@@ -414,7 +414,7 @@ impl StatefulWidget for AppWidget {
         let [v0, v1, v2] = Layout::vertical([
             Constraint::Length(1),
             Constraint::Percentage(100),
-            Constraint::Length(if state.footer.get_show_menu() { 4 } else { 1 }),
+            Constraint::Length(if state.footer.get_show_menu() { 5 } else { 1 }),
         ])
         .areas(area);