Standardise on CLOS API

This patch standardises on the class based interface to
WebKit types in cases where both object oriented and
functional interfaces are provided, resolving #13.

- WebKit types now expose properties as
  class slots, and slot-value/setf should be used
  rather than the _get/_set methods provided by the
  webkit C API (in cases where both are provided).
- WebKit type instances are constructed with
  make-instance rather than _new methods provided
  by the WebKit C API

The cookie manager and web contexts are still mainly
accessed via a functional interface, however.
Remove constant enum wrappers

Constant enum wrappers are not needed, because you
can pass :keyword values directly.
This patch resolves #15.
cookie-manager: remove what appears to be a bogus binding

not in the upstream API docs, refers to g-type initialiser,
from the looks of it.