Insert a gegl:translate node into the group layer's projection
graph. Produces identical results as the legacy projection now, but is
not yet switched on by checking View->Use Gegl.
Add implementations for GimpItem::translate(), scale(), resize(),
flip(), rotate() and transform(). Simply transform all children, the
group layer will automatically update itself. Also transform the layer
mask.
Implement the GimpProjectable interface and keep a GimpProjection
around that projects the GimpDrawableStack of our children. Propagate
the childrens' "update" signals to our own "update" signal so our
parent projection picks up all changes.