Instead, add drawable, transform, x1, y1 etc. properties. This is
cleaner and has the nice side effect of not leaving artifacts, because
the changed state of the transform tool doesn't affect the extents
calculation any longer.
And remove all the complicated handling code entirely. This makes
GimpTransformTool a lot less complex. As a nice side effect, the
preview is now always 100% in sync with the grid and handles.
