Add WebKitNavigationPolicyDecision:navigation-action object property

[joachifm]

http://webkitgtk.org/reference/webkit2gtk/stable/WebKitNavigationPolicyDecision.html#WebKitNavigationPolicyDecision--navigation-action

[Ambrevar]

Seems that we do have this: https://github.com/joachifm/cl-webkit/blob/master/webkit2/webkit2.navigation-policy-decision.lisp.
I guess this issue can be closed.

[joachifm]

I think what I meant was to add a ("navigation-action" "WebKitNavigationAction") field, navigation-type is deprecated.