Use Soup for downloads and set timeouts

Downloading via GIO uses gvfs internally. Unfortunately gvfs does not
set or handle connection timeouts, meaning that bad connectivity can
lead to a total hang.

Fixing gvfs is not easy:
https://mail.gnome.org/archives/gvfs-list/2013-July/msg00000.html

And while looking at the bug, I realised just how much overkill this
is. gvfs runs in a separate process and does the HTTP communication,
communicating back to gio over dbus, and has a few layers of abstraction.

The gio developers on IRC recommended just to use libsoup directly,
which is not hard. And we can set timeouts appropriately in order to
handle bad connectivity.

replaces https://github.com/sugarlabs/sugar/pull/57