bindings: Add gtk_binding_entry_add_signal_variant()

bindings: Add gtk_binding_entry_add_signal_variant()

This function is the replacement for
gtk_binding_entry_add_signall().

The GVariant will be demarshalled and passed to the action signal upon
binding activation. The same rules apply as used to apply for
GtkBindingArg, in that long, double and string args are now replaced by
"x", "d" and "s" variant types.