Return types from vfuncs are not converted

[BlobCodes]

(yes this is my fault)

Currently, return values from vfuncs implemented by GObject subclasses are not converted.
For example, if I want to implement the vfunc get_request_mode, I would normally assume I have to return a Gtk::SizeRequestMode. However, the current implementation wants me to return a UInt32.

I think we can copy the code used for conversion to c-types from the signal implementation (which is what I did for the vfunc parameters as well).

[hugopl]

BTW, this isn't your fault at all, the vfuncs implementation is great and it doesn't need to be 100% functional and bug free from day 0.