app: add unified interaction mode to the 3D Transform tool
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.
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