Update drag'n'drop code to match new firefox API
The API for tab drag'n'drop changed for Firefox 3.5 where canDrop() is
no longer used; provide a suitable _setEffectAllowedForDataTransfer()
instead.
The vanilla _setEffectAllowedForDataTransfer() tries to detect if the
dragged tab has been moved enough to justify a drag, but only verifies
the horizontal movement. We thus munge the screenX member of the event
before calling the function so that it thinks the tab has moved a lot.
Thanks to David Martin for the bug report