Add a GimpTransformGridTool::matrix_to_info() virtual function,
which should extract the tool-specific transformation parameters
given a transformation matrix, and the old parameter set (which is
needed in some tools, to derive the parameters that aren't encoded
in the matrix, such as the pivot point).  The transformation matrix
can be any combination of matrices calculated by the tool, and
their inverses.  Subclasses should only implement this function if
every such matrix can be mapped back to transformation parameters.
This is currently the case for all the transform-grid tools, except
for the shear tool (since it only supports shearing along one of
the horizontal or the vertical directions, however, the combined
matrix may require shearing in both directions).

When a transform-grid tool implements this function, show a chain-
button between the two transform-direction radio-buttons in the
tool options.  When the chain-button is linked, whenever the
transform corresponding to the active direction is modified, adjust
the transform corresponding to the non-active direction such that
the overall transform remains the same.

One notable workflow that this enables is transforming a layer
while adjusting a different area than its boundary, by first
defining the area while the transform-directions are linked, and
then transforming the area while the transform-directions are
unlinked.