Get rid of GimpTransformGridTool::recalc_matrix() and
gimp_transform_grid_tool_recalc_matrix(), and have
GimpTransformGridTool and its subclasses use
GimpTransformTool::recalc_matrix() and
gimp_transform_tool_recalc_matrix() directly instead.

In order to break the GimpToolWidget::changed/
GimpTransformTool::recalc_matrix() loop, add a
GimpTransformGridTool::update_widget() vfunc, which subclasses
should override to update their tool-widget (instead of doing this
in ::recalc_matrix()), and ::widget_changed(), which is called when
the tool-widget changes (and which subclasses should override
instead of connecting to the tool-widget's "changed" signal
directly.)  GimpTransformGridTool calls these functions as
necessary, instead of relying on extra parameters passed to
recalc_matrix().

Adapt all the direct and indirect subclasses of
GimpTransformGridTool to the change.

(cherry picked from commit 6a3fc6c1)