You can now pass a file:/// URL to HtmlView and it will take care of
the loading logic for you. Each Document remembers the URL it was
loaded from, which allows us to also have reload().
This patch also adds a very simple function for resolving relative
URL's into absolute ones.
Each HtmlView now has a main_frame(), which represents the main frame
of the web page. Frame inherits from TreeNode<Frame>, which will allow
us to someday implement a Frame tree (to support the <frame> element.)
You can now query Document::title() to get a String containing whatever
is inside the document's <title> tag.
In support of this, this patch adds the <html>, <head> and <title>
elements.
