Add a new 3D Transform tool, based on GimpToolTransform3DGrid,
added in the previous commit.  The tool UI provides a notbook with
three tabs, corresponding to the three GimpToolTransform3DGrid
modes:

  Camera - allows setting the primary vanishing point, as well as
  the camera's focal length, expressed either directly, or as the
  camera's angle of view, relative to the whole image or the
  transformed item.  By default, the vanishing point is aligned
  with the item's center, and the angle of view is fixed relative
  to the item; this essentially means that each item is transformed
  using a local perspective, independent of its position and size
  relative to the image.  A global perspective can be achieved by
  using a common vanishing point and focal length (or an image-
  relative angle of view).

  Move   - allows moving the item using X, Y, and Z offsets.

  Rotate - allows rotating the item using X, Y, and Z Euler angles.
  The order of rotation of the different axes can be controlled by
  a set of numbered buttons next to the sliders, and the rotation's
  pivot can be controlled using a pivot selector.