More work on the display and control of transfers.

* Hide redundant downloads from the transfer display. It seemed simplest
  to keep the behavior of queuing downloads from every remote that has a
  file, rather than going to some other data structure, but it's clutter
  to display those to the user, especially when you often have 7 copies
  of each file, like I do.
* When canceling a download, cancel all other queued downloads of that
  key too.
* Fixed unsettting of the paused flag when resuming a paused transfer.
* Implemented starting queued transfers by clicking on the start button.
* Spent a long time debugging why pausing, then resuming, and then pausing
  a transfer doesn't successfully pause it the second time. I see where
  the code is seemingly locking up in a `throwTo`, but I don't understand
  why that blocks forever. Urgh..
