let's move typedef for Vtab from public to private

libgdome/*.h has lines of typedef struct _SomethingVtab SomethingVtab;
but no more vtab matter found on public header file.
every other vtab codes are in private header.
why the only typedef in public?

I think it'll be better to move them into private code,
e.g. like typedef struct _Gdome_evt_Event Gdome_evt_Event; is in there.

it's better to track codes, I think.