Add method to create a "this" expression from GObject types

[jf2048]

cc @SeaDve

Can be useful for some expressions, makes it so the expressions example could also be simplified as:

let metadata_expression = gtk::ListItem::this_expression("item")
             .chain_property::<Note>("metadata");
// ...
 metadata_expression
            .bind(&last_modified_label, "label", Some(list_item));

Not sure if it's worth creating an extra example for this?

[SeaDve]

Not sure if it's worth creating an extra example for this?

If it is possible, maybe add it on the expressions one

[jf2048]

I changed one of the expressions in the example to use a this binding.

[SeaDve]

I think the chain on the title_binding could be just one chain, so maybe remove the metadata_expression variable and chain it directly

[jf2048]

This can wait until #714, there are some conflicts because both edit the same part of the example.