downloader: Correct documentation comment
downloader: Move downloading to download class

The Downloader got rather complex lately. It is time that the Download
object takes the part of downloading things while the Downloader only
manages several downloads.

This widely extends the (now abstract) Download object so that it
handles construction of temporary files and the await_download
functionality. (This time: thread safe.) The actual download is done by
the subclasses.

This approach allows extending this download system later with other
download methods (e.g. samba file shares among others) without changing
anything outside the download.vala file.