Add implementation to enable downloads.

This implementation will download files to the standard
download directory and assign unique file names should
there be a file with the same name existing or being downloaded.

It should be replaced with some more sophisticated code that also
takes into account settings and user choice possibly.

For now, it is only added to make downloads work at all.

Known issue: We do not get notified about completed downloads even though
we are observing the DownloadItem objects.

Change-Id: I2b52246635d3fd1358edc6b648d470ff821bc9f1
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>