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.
Right. It's a bit confusing, because xwebbrowser is a wrapper, and there is also an interface. The interface is used to make a browser usable by the wrapper, not by clients of the wrapper (at least in this case).