Drag panes by drag handle

[martinmcclure]

This issue replaces the remainder of #43.

The current small circle that is the leftmost element of a pane header is a placeholder for a handle by which the pane can be dragged to another location within the explorer window, or to another explorer window in the same taskspace. Also, dragging an inspector pane onto an inspector field should assign that instance variable to the object represented by the pane. (This action, like most others, must be undo-able.) Drag and drop is not yet implemented in Spec2, so none of this works today.

The drag functionality should work whether or not the pane is linked. Dragging a linked pane to a new location will unlink it, but dragging to assign in an inspector will not.