feat(tasks): Implement task list

[Hanziness]

This PR adds a task list to the application.

Main features

• can be turned off entirely from the settings (note the task list is disabled by default, for now)
• separate tasks for work, break and long break sections
• tasks have modifiable priorities (higher priority tasks appear before lower priority ones)
• tasks can be inactive, active or completed
• editing is only allowed before the timer starts
• completing tasks is only allowed while the timer is running (or is in "completed" state)
• only relevant tasks are shown while the timer is running and only the top n tasks are displayed
• tasks are persisted in localStorage next to the settings