Only allow drag images for canvas drags

Only allow drag images for canvas drags

The outline view "reuses" the drag source listener for the canvas,
doing some tricks with mouse events to make it look like the drag is
originating within the canvas. However, the image previews of drags do
not work well in this case. Therefore, stash data on the drag souce
such that the drag source handler can only do image previews when the
drag source is really the canvas instead of the outline page.

Change-Id: I919264f001fa232053a0b9831ebc32ee22beb877