Issue #10765: Copy NDE filters in layer group children

This patch attempts to resolve Issue #10765 (closed).

The child layers of layer groups are copied over in gimp_group_layer_duplicate (). Prior to this merge request, the NDE filters attached to the children were being lost there.

The actual copy code is similar to how individual layers are handled. However, the current GimpDrawableFilter code requires layers to be attached to an image before they can be added. Because the new children layers are attached to the group layer instead at this point, the check for gimp_item_is_attached () failed, preventing the copy.

gimp_viewable_get_parent () was added as an OR condition to that check to fix this. I'm not sure if this is the correct approach however, so feedback is appreciated.

@brunolopesdsilv, this one's for you. 😄 I've tested that copy/paste, duplication, and exporting work at least two groups deep, but since you use a lot of groups in your work I'd appreciate any testing you have time for.