Handle WebKitWebView create-web-view.

In some cases a new window is signalled using create-web-view rather than new-window-policy-decision-requested and so we need to handle it to emit the correct new window events.

Fixes #15447.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74804 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775