new background downloader

Some old work on a proper background downloader with threads and stuff.

todo

• the idea is downloads are supposed to be flaky
• currently harded to fail with some probability and take random amounts of time.
  • REMOVE BEFORE MERGING
• display a placeholder image while things are downloading
• separate code into its own file/module
• some reasonable interaction in a threadsafe way with the pagecache.
• better placeholder graphic: filed #2357
• try at most say 3 times before failing outright: don't like the idea of buggy stuff endlessly redownloading in the background
• ctrl-R dialog does blocking downloads of any pages we don't yet have (same is in main)
  • could port Ctrl-R dialog for lazy download, or leave for a future MR (probably defer, that code is hard to edit, #2355)
  • port other pop-ups to lazy downloads (or again, leave for future MR) (Filed #2356)
• this does not try to fix/change anything about uploads.
• poolsize should be fixed at say 2
• is there some lock state where Marker doesn't get a thread?
  • is the threadpool reclaining the marker thread?
• stopping:
  • Downloader has a "clear queue" method
  • Downloader has a stop/wait method
  • Marker versus Chooser: who should be waiting? What about shared downloader? filed #2359 (closed)