Add a new GimpFilterTool::region_changed() virtual function, which
gets called whenever the filter region changes, either due to a
change to the tool's "region" option, or a change to the image
mask.
Override GimpFilterTool::region_changed() in GimpOperationTool and
GimpOffsetTool, instead of listening to a change to the "region"
option in GimpTool::options_notify(), so that the tools are
properly updated when the image mask changes.
(cherry picked from commit 066827e23c)
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)
