Issue #40 - Layer offset tool

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)

app/dialogs/dialogs.c

@@ -236,6 +236,7 @@ static const GimpDialogFactoryEntry entries[] =
   FOREIGN ("gimp-hue-saturation-tool-dialog",      TRUE,  FALSE),
   FOREIGN ("gimp-levels-tool-dialog",              TRUE,  TRUE),
   FOREIGN ("gimp-measure-tool-dialog",             TRUE,  FALSE),
+  FOREIGN ("gimp-offset-tool-dialog",              TRUE,  FALSE),
   FOREIGN ("gimp-operation-tool-dialog",           TRUE,  FALSE),
   FOREIGN ("gimp-posterize-tool-dialog",           TRUE,  FALSE),
   FOREIGN ("gimp-rotate-tool-dialog",              TRUE,  FALSE),