Built-in download feature

Downloading via an external tool poses a problem: You have to pass the
current "web context" from the browser to your tool. This context
comprises cookies, the referrer, the user agent and information about
HTTP basic auth. With some effort, you can pass most of this to your
tool -- except for HTTP basic auth.

tl;dr: Downloading via wget is pretty complicated.

With this commit, WebKit handles the downloads. What's missing, are some
GUI elements to monitor and cancel downloads.