Reorganize tool modes to be { ADD_TRANSFORM, MOVE, REMOVE }, where
ADD_TRANSFORM is the default and allows to add handles *and* transform
the image in one click-drag. MOVE moves handles without transforming
(shift) and REMOVE removes handles (control). Also improve cursors to
accurately show the result of a click.
Add new tool GimpHandleTransformTool which allows to freely place up
to 4 handles on the image, then move any one of them, which transforms
the image so that the remaining handles keep their position.
Did quite some cleanup on the code before pushing --Mitch
