Fix blank About Boxes & allow link-following in external browser

I changed the meaning of XWebBrowser's AllowNavigation=false to mean: allow first page to load, but after that, open links in the system browser. The winforms meaning, is in contrast: prevent loading even the first page. If manually disabled only after that load, then ignore all link clicks.

Some previous commit left us with initial loads that failed, leading to, for example, empty About boxes in in all LibPalaso clients.
This property was previously a way to get at the native AllowNavigation of the browser being wrapped. We are no longer making use of that property, but instead dealing with navigation issues at the wrapper level. Unless a client was bypassing the wrapper and going directly to the AllowNavigation property of a browser through this interface (which would make no sense since the wrapper previously just passed it on), it will have no effect on existing code.