Mint 19 Cinnamon: Download continues when software manager is closed

[jeroenst]

When in the software manager an install process is downloading packages and the software manager is closed the download process continues. On slow data connections this causes problems of leaving the user with a slow internet experience and no way to cancel the download.

The software manager should kill the running download progress when closed.

Also a cancel button should imo be included when a download in the software manager is running.