Create Dialog Relm Widget

[torkleyy]

Hey @antoyo, thanks for this library! Do you think it would make sense to add an example showing how to create a gtk::Dialog using the view! macro?

In particular, I'm wondering how to replicate the Dialog::new_with_buttons function. I think the default constructor is kind of deprecated.

[antoyo]

So, the Dialog widget is a bit special and it will require a new Dialog relm widget to make it work.