Add support for internal header and VAPI

[powerwaremediacore]

Vala compiler allows to create internal symbol headers and corresponding VAPI, used when extend a class with protected fields you are allowed to access.

Is required to port Gee to Meson, because it uses:

--internal-header and --internal-vapi to generate internal symbols.

Pull request with a patch, but with some issues when try to create VAPI for internal symbols is:

#1780