Replace builders with generated traits on ObjectBuilder

[jf2048]

Alternate solution to #1391. Tested with gtk4 it removes around 40,000 lines of code.

I had to remove builder_postprocess too. For that I would like to somehow come up with a way to fold the setup into FromGlibPtr and FromValue.

[jf2048]

I think something needs to be done in gtk-rs-core about the missing builder_postprocess first

[jf2048]

Sorry meant gtk3-rs and gtk4-rs, those seem to be the only places using it

[sdroege]

@jf2048 @bilelmoussaoui What's the plan/status here?

[jf2048]

It is a real pain to modify that wrapper macro to do what we need to do here

[jf2048]

https://github.com/gtk-rs/gtk-rs-core/issues/916