Rework the queue so in-progress/completed/failed entries remain visible

[ssokolow]

It really shouldn't be necessary to scroll through the progress window to identify failures.

Instead, I'll want to add a column on the left which indicates one of four different states:

• Queued (blank)
• In-progress (not sure. throbber? )
• Completed (gtk-ok from system theme: )
• Error (dialog-error from system theme: )

The throbber is provided by GTK+ but the chosen icons are not universally installed, so I'll want to bundle a fallback. Tango is in the public domain and provides a suitable error icon but I may have to add another "...except for..." to the licensing info and bundle an LGPLed or GPLed icon for OK.