Add support for "closure user_data" and "closure scope call"

[sdroege]

We should be able to autogenerate trampolines and safe bindings for all functions taking "closure scope call" (e.g. gtk_list_box_selected_foreach() https://github.com/sdroege/gtk-rs/blob/24c4c8305c235592c49bf2018d0f6e0afdeeb4d7/src/list_box.rs#L76-L102) and generally closures that have a user_data and destroy_notify (e.g. gtk_list_box_bind_model(), https://github.com/sdroege/gtk-rs/blob/24c4c8305c235592c49bf2018d0f6e0afdeeb4d7/src/list_box.rs#L38-L73).

The trampolines are basically the same as for signals, "scope call" closures can be FnMut and directly passed via the stack, other closures need to be stored (just like for signals) in a Box and need to be `'static'.

The Send / Sync bounds can be inferred from the corresponding bounds on the object (but like for signals an override would make sense).

[sdroege]

@GuillaumeGomez @EPashkin Do you see any problems here?

[GuillaumeGomez]

If the result is the same as the one in the linked PR, then it seems all good to me.

[EPashkin]

IMHO good idea