Clearing all completed tasks doesn't reset "Done" button
Seleting "Clear completed tasks" doesn't clear the "Done" button. This may be an issue from #136 (closed).
How to reproduce:
- Set several items as done (clicking the check button)
- Click menu -> Clear completed tasks
Result: The "Done" button isn't hidden, and shows the same previous count. Also, checking more items increments the "Done" button item count.
This was tested with all extensions disabled.