gschem: Fix search & replace garbage.

Ed likes to do search and replace over the whole codebase and then
commit the changes without actually looking at them.
scheme-api: Provide integration of SCM values into GType system.

Allow GObjects to be created that directly accept Guile Scheme values
as properties, without having to mess about with SCM_PACK() and
SCM_UNPACK().
scheme-api: Make EdascmHookProxy use EDASCM_TYPE_SCM.