Imageset system floods ACP log with errors when not using localised images.

The imageset system currently creates an "Imageset misses en localisation » prosilver" entry in the ACP log on every page load when the style is missing a localised imageset. The imageset system should take into account that a style author may decide to simply use text rather than create localised images. In this situation, the author is forced to include an imageset.cfg for the localised imageset that includes an image entry that isn't used in order to avoid flooding the ACP log.

My suggestion would be to create a new setting for the main imageset.cfg, such as use_localised_imageset = false, that would be checked before creating the entry in the ACP log. This would allow the author to avoid including unnecessary directories altogether.