Mark generic interfaces as being covariant

[stof]

This makes them easier to use when writing a method that can accept any data inside the PagerfantaInterface argument. And the interface are only using the template type in covariant positions.

[stof]

@mbabker do you have any ETA for when the next release will be done with the few type improvements I submitted ?

[mbabker]

That would be right now.

[stof]

thanks