Add a "Unified interaction" option to the 3D Transform tool, and a
corresponding "unified" property to GimpToolTransform3DGrid.  When
active, all three interaction modes of the grid (camera, move, and
rotate) are available simultaneously, regardless of the active
dialog page.  In this mode, the inner and outer regions of the item
are used for moving and rotation, respectively, and the vanishing-
point is controlled through through a handle.