Implement GObject classes for webkit types

Now WebKitWebView, WebKitWebContext, WebKitCookieManager,
WebKitDownload, and WebKitFindController are implemented
as g-object classes, with the appropriate superclasses.

This change permits adding GObject signal handlers to
webkit objects, e.g., "load-changed" for WebKitWebView.
Migrate all foreign functions to GObject classes

All webkit functions now accept and return gobject-class
values.
Existing users of the binding should not have to change
anything.
Implement WebKitLoadEvent enum via gobject

GObject needs to know about this e.g., for the "load-changed"
signal emitted by WebKitWebView, so we need to use
the define-g-enum wrapper rather than use CFFI directly.
Merge branch 'gobject'