Derive GimpUnifiedTransformTool, GimpPerspectiveTool, and
GimpHandleTransformTool from GimpGenericTransformTool, eliminating
their common logic.
Remove gimp_transform_matrix_handles(), which is no longer used.
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