New and with_type methods as in gtk-rs-core

[ranfdev]

In general, we should provide typed methods when possible and with_type variants.

Example: Widget::ancestor(widget, type) (which currently returns a Option<Widget>) could become Widget::ancestor::<gtk::Label>(widget) and directly return a typed Option<Label>.

[bilelmoussaoui]

Would be nice to gather a list of methods that could make use of such changes.

[jf2048]

I counted these:

gdk4::ContentFormats::for_type
gdk4::ContentFormats::contains_type
gdk4::ContentFormatsBuilder::add_type
gdk4::ContentProviderExtManual::value
gtk4::DropTarget::new
gtk4::WidgetExt::ancestor

[bilelmoussaoui]

More can be found in https://github.com/search?q=repo%3Agtk-rs%2Fgtk4-rs%20%20glib%3A%3Atypes%3A%3AType&type=code