There's no need to be throwing all this up-front known crap into a runtime constructed dictionary.

If the widget has a "clicked" signal, there should be a typedef of the callback function appropriate for that signal and the widget struct should have a clicked_cb member for a list of callbacks of that type. Then there should simply be a unique function for installing the callback on that signal on a widget. At least then the compiler can tell you when you're mixing up callbacks<->signals for API-defined signals. It's not even complicated.