Add functionality for generating `UrlRequest`s explicitly

[cacay]

Currently, a UrlRequest is generated when the user clicks on a link. There should be a function similar to Navigation.pushUrl except that it goes through application.onUrlRequest. This way, we can delegate to application.onUrlRequest from onClick and other handlers.

Here's my use case. I wrote a wrapper around Browser.application that, given the old URL and the new, decides whether to use pushUrl or replaceUrl. That way, all the logic of handling history is collected in one place instead of scattered across the application. This works fine for links, but not for handlers where I have to do the decision locally. Worse, since the old URL is maintained in the wrapper and not in the application, I can't implement this even if I'm OK with duplicating code.