HtmlView will now invoke the on_link_hover hook when the cursor enters
or leaves a DOM node that has an enclosing link element.
This patch also updates the meaning of Node::enclosing_link_element()
to find the nearest HTMLAnchorElementAncestor *with an href attribute*.
The layout root is now kept alive via Document::m_layout_root.
This will allow us to do more layout-related things inside the inner
layer of LibHTML without reaching out to the HtmlView.
I'd like to keep HtmlView at a slightly higher level, to prevent it
from getting too complex.
This patch also fixes accidental disconnection of the layout tree from
the DOM after doing a layout tree rebuild. ~LayoutNode() now only
unsets the DOM node's layout_node() if it's itself.
For now this is simply a counter+hook exposed by ResourceLoader and
shown in the Browser status bar.
This is not very nuanced, and it would be nice to expose more info so
we could eventually do something like a progress bar.
