This addon is based on the patch I wrote for bug 589401.

1. Dragging back out not implemented
Currently, this only allows dragging into Panorama, and doesn't support dragging
back out to the rest of the browser (even on the same drag that entered).  

This is due to the complication of handling when the Panorama button is in the
same location both inside and outside of Panorama.  In that case, there is a
tendency to "bounce" between the two states.  

The remedy would be to fiddle with the event listeners, but I haven't found a
robust way to handle that at present, due to the fact that there isn't an easy
way to tell if a drag ended unless we are the source node; we _can_ get the
source node via dataTransfer.mozSourceNode, but it won't exist for external
drags.  

Suggestions needed for handling that case.

2. Dragging tabs between windows not implemented
If a tab does not have a _tabViewTabItem (which there doesn't seem to be when
dragging between windows), it can't be dropped.  Apparently tabs don't pass
alternate data (eg, the URI in a text form), so it will simply balk.

Solution might be to construct a new TabItem or see how that data might be
made available between windows so that simply turning it on would work.

3. Replacing the drag with the tabitem
Assuming [1] and [2] cannot be resolved, it would make sense to try to replace
the drag with the tabitem once in Panorama.  At that point the true drag would
be cancelled and only the Panorama drag would be used.  It would make that case
simpler, but it wouldn't necessarily handle the non-tab cases.

For the non-tab cases, it's possible that the new tab could be created and then
replace the drag with it, though.