update-manager should have download speed limit

[Wojciech Bartłomiej Chojnacki]

update-manager doesn't have basic functionality - setting the download speed limit.
It's very anoying when a security update is downloading in the background and it's slowing down the overall connection speed.

Also, when few big updates are available for download, then it's very irritating if we want to start downloading them now and we don't have too fast connection speed, so when we're using Internet for doing other stuff, we're forced to wait as long as updates are not downloaded.