Decreace opacity of a completed task.

[arshubham]

Currently on looking at the whole agenda the completed task stand out more than they should and steal focus from incomplete tasks. This in conjuction with #81 will make experience much better.

[be1]

As long as the window background is filled with a plain color, decrasing opacity of a task item would sort of "gray out" it, this is a good idea. I will try to fix that issue soon. Thanks for this idea.