Use Task.set_status to mark tasks as done or active

[gycsaba96]

Fixes #1180

Somehow, setting the task done was triggered twice: in TaskEditor.change_status and in MainWindow.on_mark_as_done. Since MainWindow.on_mark_as_done also used Task.toggle_status, the effect was reversed.

Some remarks:

• I don't clearly see why was the set_status triggered twice, but since fixing MainWindow.on_mark_as_done fixes the problem, I would say it is a question for another day.
• Dismissing tasks may have similar problems, but I couldn't yet reproduce the bug (see MainWindow.on_dismiss_task).

[SqAtx]

Thanks for looking at my bug!

I'm not intimate with that part of the code and I'm just looking at the diff, but it looks like you're making it so that the code does what we mean! It was making assumptions that it didn't need to make. LGTM!

[diegogangl]

TBH I don't remember why I used toggle_active() instead of setting the status. Probably porting something from a previous version? Anyways, this LGTM. Thanks!