multithreaded Download class, used in open_ui

each of the 4 downloads is in its own thread, if the download is calceled before it finishes, it finishes downloading anyway and the download thread cleans up everything at the end (TODO: apparently canceling downloads in the middle isn't really so hacky)
force close when downloads are canceled, default keepAlive to false (wasn't even needed ...)

download thread still cleans up stuff itself