Drop gcr-gtk3 and gcr-gtk4

gcr historically consisted of 2 high-level parts: gcr-base and gcr-ui. gcr-base contains the core classes and interfaces to deal with crypto-related items (e.g. GcrCertificate), while gcr-ui contained GTK widgets to show those items (e.g. GcrCertificateWidget).

Now: with the move to gcr4, it's becoming more and more clear to that this isn't really a path forward:

On one hand, GTK4 has transitioned from a platform toolkit (usually GNOME was the primary target) to one that allows you to build your platform on top (e.g. libadwaita, libgranite, or your very own). Kepeing that in mind, having "GTK-based" widgets for use in general purpose doesn't really make sense, since it will always look out touch on platforms

On the other hand, widgets are usually more faster-moving targets in both looks as well as API than an actual library, so in practice gcr-ui has a different lifecycle than gcr-base.

Finally, @tintou has been doing an awesome effort to implement an API that allows consumers to write their own widgets, without having to deal with asn1 decoding etc. At this point, I think the certificate widget is likely the only widget we're seeing interest in.

As such, this commit drops gcr-gtk3 and gcr-gtk4 as libraries. There's still a gcr-viewer debugging tool as a troubleshooting/debugging tool, that's it.

See #100 (closed) for the related discussion.