Pane drag handle should also be unlink button

[martinmcclure]

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 a 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.

This handle element should take on more functions.
Visually, it should change appearance to indicate whether the pane is a linked pane, or is independent (unlinked).
If the pane is linked, clicking on the handle should unlink the pane from its parent pane and change appearance.
Also the drag functionality described above should work whether or not the pane is linked. Dragging a linked pane to a new location will also unlink it, but dragging to assign in an inspector will not.

[martinmcclure]

The unlinking part of this issue was implemented in #114. The remaining drag/drop functionality is recorded in #115; closing this issue.