When decomposing, the plug-in sets the filename without changing the
extension. However, gimp_image_set_file requires the filename to be
an .xcf file.

To fix this we simplified `generate_filename` to always add an
.xcf extension.
While doing this we also fixed a critical that happened when no
filename was set (new image), by testing `gimp_image_get_file` for
being NULL first.
When no filename was set it also didn't use the type when
config_as_layers was set, this is also fixed.