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.
Actually it's always about:blank in my tests; the Url != null is just defensive programming since this is supposed to work with multiple browsers and I can't be certain they would all have "about:blank" initially.