We can't rely on g_object_unref() in halt() for breaking all property
GBindings between the tool options and GimpToolRectangle, because we
might be in the middle of a signal emission which also refs and keeps
the rectangle alive until the last callback returns. So we had
dangling rectangles interacting with tool options.

Remember all bindings in a list, and break them explicitly when we
shut down the rectangle in halt().

Also, forgot to unset the display's highlight in the rectangle
selection tool.