cl-wekit-dom: add init

This is a copy-paste from webkit2.init.
At some point, figure out a neater setup here;
this code should be shared between these components.
soup/messsage-headers: vanilla bindings
soup/init: fall back to unversioned .so
Implement proper translate-webkit-class-name

Uses cffi:translate-camelcase-name instead of the
hacky medial->upper.
macros (define-webkit-class): use translate-webkit-class-name