We are going to use this in other object types as well

This allows plugins to provide custom interfaces on proxies

* make the registries available as dynamic interfaces of the core
* improve a bit the C API of WpObject & WpInterfaceImpl

the GType cannot be in statically allocated data,
since it's not known at compile time

glib log domains are not like the gstreamer ones, where you can enable
many of them with a wildcard, therefore it is not particularly useful
to have different ones per file