Needed because a group layer's projection can have a different type
than its image, at least at some pathologic moment during image type
conversion. But even if it didn't, it's cleaner this way anyway.

* app/core/gimpprojectable.[ch]: add the new vfunc plus public API.

* app/core/gimpprojection.c: use it instead of using the type of the
  image returned by gimp_projectable_get_image(). The get_image()
  function is now unused in the projection and only needs to stay
  because the projection is also a GimpPickable.

* app/core/gimpgrouplayer.c: implement GimpProjectable::get_image_type().