codegen: Allow usage of gtype structs inheriting non-gtype struct
codegen: Allow usage of gtype structs inheriting non-gtype struct

The behavior of type_id is different, but get/set/take/marshaller aren't
as specific as type_id. Therefore this patch enhances without breaking.

Fixes bug 669604.

frida/vala,flobrosch/vala-fixes,severinh/vala,GNOME/vala,severinh/vala,frida/vala,frida/vala,rvu95/vala,Distrotech/vala,playya/vala,jessevdk/vala,frida/vala,flobrosch/vala-fixes,jessevdk/vala,spidermonkdust/vala,GNOME/vala,jessevdk/vala,playya/vala,GNOME/vala,spidermonkdust/vala,spidermonkdust/vala,playya/vala,frida/vala,rvu95/vala,tjol/vala-tjol,xdave/pvala,tjol/vala-tjol,severinh/vala,Distrotech/vala,GNOME/vala,Distrotech/vala,rvu95/vala,flobrosch/vala-fixes,tjol/vala-tjol