also add "GType old_type" to GimpItem::convert() so implementations
can do things depending on the type of the original item.

In gimp_layer_convert(), if the original item is also a layer, and
color management is not off (with a FIXME because this is the wrong
check), pass convert_profile = TRUE to gimp_drawable_convert_type().

There is no color profile conversion anywhere behind this, this is
just an API change commit.