[Vala] control VAPI output

[xiaohu <xiaohu417 gmail com>]

hi:

I have a vala program, which support plugin, so i need expose some
interface by a vapi file to external.

I use 'internal' accesser instead 'public', this is work well for most
case, but there still has a problem if I use GtkBuilder.
If I declare my gtkbuilder object's callback as public, them will
exposed in VAPI, if I declare object's callback as internal,
the vala compiler will generate warning: method `...' never used.

Is there has way to exact control which method can be expose?

thanks.