Add the ability to have circular references to Widgets (or at the very least Window)
For Features such as CaptureWidget on `SearchEntry` (as discussed [here](https://github.com/can-lehmann/owlkettle/pull/109#issuecomment-1762132276) ) and I think others as well, it is mandatory to be able to have circular references.

Without those implemented, the full featureset of all GTK widgets can not be provided.

I'd like to have an issue about this simply so we can keep this in mind and have somewhere to track possible progress against etc.

Though honestly, how this could possibly work is entirely beyond me.