If Error is triggered during the upload, cancel the upload, before proceeding to the next file.
Use single quotes for event names.

For consistency.
Fix for event propagation.
FilesAdded is asynchronous already, no need to detach consequent QueueChanged and Refresh that are triggered from within the default handler.
Get rid of unnecessary closures.
Reset the retries counter after a successful transmission.
Fix: Queue Widget drag/drop

The change in SHA: aa05433ae8a0bb59ebc4a13c8406437e130a4d92 caused the
Drag/Drop functionality to stop working.
Reference submodules via HTTPS. Update moxiecode/testrunner@f638fec4c438815594876e5e4f21c2184d0760be. Closes #917.