Exposure fusion should be grayed when no layer group was defined.

On est d'autant plus surpris que ça marche pas quand a vu les marques [BKT] sur les groupes de la fenêtre principale! Adding a "(no layer groups)" remarks next to the selection box would be nice, too.

This certainly applies to any of the different 2.0.5 versions ( though for Linux geeks I'm not sure this really matters a lot )