If a URL has an embedded link fingerprint the checksum
and algorithm is shown in the download prompt. After a
file has been saved to disk a warning icon will be shown
if the checksum doesn't match and clicking the icon will
explain that the file is erroneous and not open it.
See http://mdhashtool.mozdev.org/lfinfo.html
The transferbar is separated from the browser, albeit it is still
created and filled from within the browser so it is not completely
independent.
The new class now keeps a list of TransferInfo items to keep track
of transfers and queries the status from the download object.
Destroying a button belonging to a transfer is connected to
removing it from the list and cleaning up the info structure. Also
the Clear button is updated from the destroy handler.
