Add flags, enums and error to the example

[elmarco]

Flags & Enums should be easy to cover today.

Error will need some new glib-rs macro

[sdroege]

Error will need some new glib-rs macro

Can you create a gtk-rs issue for that?

[elmarco]

Error will need some new glib-rs macro

Can you create a gtk-rs issue for that?

I had a grip on it gtk-rs/gtk3-rs#367

[sdroege]

Even better!