When handling an external URI or downloading a file, invoke Torbutton's
external app blocker component (which will present a download warning
dialog unless the user has checked the "Automatically download files
from now on" box).

For e10s compatibility, avoid using a modal dialog and instead use
a callback interface (nsIHelperAppWarningLauncher) to allow Torbutton
to indicate the user's desire to cancel or continue each request.

Other bugs fixed:
 Bug 21766: Crash with e10s enabled while trying to download a file
 Bug 21886: Download is stalled in non-e10s mode
 Bug 22471: Downloading files via the PDF viewer download button is broken
 Bug 22472: Fix FTP downloads when external helper app dialog is shown
 Bug 22610: Avoid crashes when canceling external helper app downloads
 Bug 22618: Downloading pdf file via file:/// is stalling