gschem: Rename GSCHEM_TOPLEVEL to GschemToplevel

Rename GSCHEM_TOPLEVEL to GschemToplevel to match class naming conventions.
gschem: Add class cast for GschemToplevel
gschem: Add accessor to GschemToplevel for TOPLEVEL
scheme-api: Add version information to documentation.

Add info on which versions of gEDA introduced various new functions,
variables etc. to the Scheme API.

Since the next stable version to be released will probably be 1.10,
use that for now.
Remove all libdmalloc cruft.