applets/digital-clock: improve arrow key navigation support

1. Add Button ↔ TabBar
2. Add Button ↕ Event List if not empty
3. Event List → Month View
4. Event List ↕ Switch Button if visible
5. Switch Button → Month View
6. Switch Button ↕ Timezone List
7. Timezone List → Month View
8. TabBar ↔ Previous Button ↔ Today Button ↔ Next Button
9. TabBar/Previous Button/Today Button ↑ Configure Button
10. Next Button ↕ Pin Button
11. Configure Button ↕ Today Button
12. TabBar/Previous Button/Today Button/Next Button ↓ Month View
13. Month View ↕ TabBar

dclock

Ref: plasma-desktop#42 (closed)