Changed the window used to assign pictures to materials. From the new window the user can delete the picture assigned to a material. The old button for deleting picture (from the Materials window toolbar) has been removed.
Fixed a problem where some special characters (\ / : * ? ” < > |) in the material name prevented the image from being saved to disk. These special characters are now replaced by _ in the filename of the image assigned to a material. This solution is not perfect (since the same character replaces multiple invalid characters), but it is better than before … and will be improved in the near future.
