Minor improvement to changed_download_progress()

I checked WebKit's source code: webkit_download_get_destination_uri()
never returns NULL in lariza. It's because we set the destination URI in
the signal handler for the download request.

If g_filename_from_uri() really returned NULL for some reason, then it'd
be just plain wrong to announce the "suggested file name", because we
might be saving the download ... somewhere else. I doubt this function
ever returns NULL in lariza, though, but you never know.

So, now, the download manager will correctly show the URI if the
destination URI is "unknown".
Show size of downloads in megabyte

MB is good enough, I don't care about byte, kilobyte, megabyte,
gigabyte, ...

Yes, SI units.
Code style: Allow more than 80 chars to avoid ugliness

Plus, my "80 characters" only work with 4-spaced-tabs anyway. I might
rethink that.