Add a new GimpToolTransform3DGrid tool widget, subclassed from
GimpToolTransformGrid, which can be used to perform 3D
transformations.

The widget can be in one of three modes:

  CAMERA - allows adjusting the primary vanishing point by moving a
  handle.

  MOVE   - allows moving the object through dragging.

  ROTATE - allows rotating the object through dragging.

By default, controlling the transformation through dragging applies
to the X and Y axes.  Holding Shift (or setting the "constrain-
axis" property) restricts the motion to only one of the axes.

For the MOVE and ROTATE mode, holding Ctrl (or setting the "z-axis"
property) allows controlling the Z axis instead.

For the same modes, holding Alt (or setting the "local-frame"
property), applies the adjustments in the object's local frame of
reference, instead of the display's global frame of reference.