Plumb progress through DBus signals

As per spec, no progress will be sent while downloads are occurring which are only used to decide if an update is available. When the actual update files are being downloaded, progress from the u-d-m will be plumbed through (after conversion, see below) to UpdateProgress signals as per the s-i D-Bus spec. Also as per spec, we always start by sending an UpdateProgress(0, 0) signal, so you are guaranteed to at least get that. The rest is up to u-d-m.

Note that u-d-m gives us different progress values than the u/i wants from s-i. u-d-m gives us bytes-received and total-bytes. s-i gives percentage (as integer from 0 to 100) and eta. We cannot currently calculate an accurate eta, so again as per spec, we'll always send an eta of 0, although the percentage will be correct, floored to an integer. If the u/i wants an accurate (as possible) eta, let's open a separate bug on that.