At all filter:
1. create
2. edit
3. delete
4. complete
5. activate
6. complete all
7. clear completed

At active filter:
1. create
2. edit
3. delete
4. complete
5. clear completed

At completed filter:
1. delete
2. activate
3. clear completed

Additional edit operations:
1. cancel edit by press esc
2. delete a task by inserting an empty string
3. confirm edit by press teb
4. confirm edit by click outside

General:
1. “items left” counting

Switching between filters:
from All to
    Active
    Completed
from Active to
    All
    Completed
from Completed to
    Active
    Completed