Enhance performance of lots of quick tasks

[aonez]

As @sn0wyfall stated in https://github.com/aonez/Keka/issues/773#issuecomment-778709642, lots of quick tasks can make Keka unstable and laggy.

[quantumgolem]

By the way, my comment could also apply for larger zip files. If you have 100 zips of 100MB+ each, you'll get a notification bounce 100 times. I think it would be much better to have the icon bounce just once, when all of the zips are finished unzipping.