Signals with optional parameters

[EPashkin]

Part of #199
ListBox: row-selected
Widget: hierarchy-changed, parent-set

For closure Fn(&ListBox, Option<&ListBoxRow>) + 'static> generated generated bad code.
Not sure if it need be fixed in gir or in glib\gtk.

unsafe extern "C" fn row_selected_trampoline(this: *mut GtkListBox, row: *mut GtkListBoxRow, f: gpointer) {
    callback_guard!();
    let f: &Box_<Fn(&ListBox, Option<&ListBoxRow>) + 'static> = transmute(f);
    f(&from_glib_none(this), Option<ListBoxRow>::from_glib_none(row))
}

[EPashkin]

Added partial fix

[EPashkin]

Seems gir generated working code, I close this.