app, icons: add 3D Transform tool
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.