Add a new Offset filter tool, as a front-end to gimp:offset. The
tool replaces, and provides the same interface as, the drawable-
offset dialog, while also providing live preview and on-canvas
interaction.
Note that we don't simply use a custom propgui constructor for
gimp:offset, since we need a little more control.
(cherry picked from commit 3a4a00c71e)
ca4612cc43