Add component task-cell
Add taskCell and incompletedBadge counter

Represent tasks and count how many are not yet done.
Indicate completed tasks

They should appear visually distinct from tasks that need to be completed.
Add Clear Completed Button

To clear away completed tasks
Toggle enabled state of clear compelted button

It should only be enabled when there are completed tasks,
though actually calling the clearCompleted is still a no-op.