gio: Add Settings::bind_with_mapping() binding

[ids1024]

Using boxed Fn mainly to have a reasonable way to pass None.

[ids1024]

Now this commit is using:

get_mapping: Option<Box<dyn Fn(&glib::Variant, glib::Type) -> Option<glib::Value>>>,
set_mapping: Option<Box<dyn Fn(&glib::Value, glib::VariantType) -> Option<glib::Variant>>>,

Which seems like a reasonable Rust function signature, matching the capabilities of the C API. Most implementations will probably use closures ignoring the Type and VariantType arguments, but that's easy enough.

[ids1024]

Updated to use a builder pattern. Still doesn't seem like a perfect API, but I suppose I'm at least happy to not be writing Some(Box::new(...)) when using it.

[sdroege]

Updated to use a builder pattern. Still doesn't seem like a perfect API, but I suppose I'm at least happy to not be writing Some(Box::new(...)) when using it.

Yeah writing the Some(Box::new()) in user code is always annoying, so this is a huge usability improvement IMHO.