Just in time for Serenity's 1st birthday, here is the <blink> element!
This patch adds a bunch of different mechanisms to enable partial
repaints of the layout tree (LayoutNode::set_needs_display()))
It also adds LayoutNode::is_visible(), which can be toggled to prevent
a LayoutNode from rendering anything (it still takes up space though.)
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.)
This also fixes another bug with inline wrappers. Namely,
we should only add inline wrappers if a block node has
both non-block (inline or text) and block children.
