Add ability to open external links in defaut browser

[Jason Gerard DeRose]

One of the nice things about building UI with web tech is the ability to use hyper linking both to link internally within the application and to link externally onto the web.

But they need to be handled differently... internal links are used to move between different screens in the application, and should rightly change the WebKit location. But external links should not as we don't want to replace our native application with the contents of a remote web page... in this case, we want open these links in the user's preferred browser.

So this bug tracks this feature. It adds a new 'open' signal to CouchView (which internally is driven by the WebView 'navigation-policy-decision-requested' signal with some CouchView logic on top).

And the BaseUI class now connects to this signal and opens the URL with xdg-open. We may want to change this to be opt in for apps later, but for now we might as well play with it.