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.
It doesn't make much sense to navigate an external browser to about:blank, but then, it doesn't make much sense to navigate the one we're wrapping there and then never again be able to take it somewhere else, either. Url != null achieves the "allow first time" thing, but it might be worth commenting on the reason for the about:blank check.