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.
This was a workaround to be able to build on case-insensitive file
systems where it might get confused about <string.h> vs <String.h>.
Let's just not support building that way, so String.h can have an
objectively nicer name. :^)
