Replace GimpTransformTool's 'drawable' field with an 'item' field,
and have GimpTransformGridTool set it to the active item, to which
the transformation is applied, during its initialization.  In
gimp_transform_tool_get_active_item(), return the value of the
transform tool's 'item' field, if not NULL, instead of the image's
active item.  This makes sure we apply that transform-grid tools
apply the transformation for the item for which they were
activated, even if the image's active item has changed.