Split common.h.templ into private and public headers and "seal" the private one

[martin-ejdestig]

As e.g. GLib (and GTK does). Only <glib.h> is supposed to be included and all other headers are guarded by:

#if !defined (__GLIB_H_INSIDE__) && !defined (GLIB_COMPILATION)
#error "Only <glib.h> can be included directly."
#endif

commoh.h.templ currently contains *TypeWrap and Error if any error annotations are specified in introspection XML. *TypeWrap is not meant to be used externally (if I have understood things correctly) while Error is.

Could put *TypeWrap in common_private.h.templ and require FOO_BAR_OR_SOMETHING to be defined before it is included. Error would still stay in commoh.h.templ.

[martin-ejdestig]

Could also keep everything in a single header and just have a section of commoh.h.templ wrapped in an #ifdef guard.

Would avoid adding yet another file that user has to specify when invoking generator from build systems. Also a less intrusive change.