Add NDE support for group layers
This patch attempts to add "adjustment layer groups" by allowing GEGL filters to be applied to layer groups.
It mostly consists of removing the current restrictions on adding filters to groups (with the exception of a few non-GUI filters like invert) and disabling the "Merge Down" button on the NDE pop-over when a group layer is selected.
Video: https://fosstodon.org/@CmykStudent/111997898335834638
Edited by Alx Sa